Turn on your pc if you have password log on wait for windows to start up even if you can not see anything, then press ctrl+Alt+delet this is bring up task manager then you click on file then new tsak type restore it should bring up a another page click on restore and do a restore a week or two week ago , choose a date that your pc been working fine
If none of this work you have to format it

•
First things first ... let's get you a bootable system.
Boot in safe mode - press F8 when the Windows loader presents you with the 30 second list of choices, and then select "Safe Mode" as your boot option.
Right click on the desktop and click Properties, and then on the Settings tab.
Now click and drag the Screen Resolution slider to the far left ... you want the smallest settings that your screen will support. Typically that'll be 640x480, or 800x600.
Also change the Color Quality dropdown to 256 colors, or whatever is lowest. (You probably don't need to go as low as 16 colors, if that's presented as an option.)
Now, click on the Advanced button, and then the Monitor tab of the resulting dialog box. Change the Screen refresh rate to 60 Hertz.
OK your way back out, and reboot your machine. If these setting were the source of the problem, it should now be able to boot normally. You may not like the screen settings, but at least you now have a place to start.
"Start experimenting with the settings to see what does, and does not, work with your monitor."
If the display is still blank or gibberish, the my recommendation is to once again boot into Safe Mode, possibly Safe Mode with networking, and update your video display drivers. It's likely that they are incorrect or otherwise broken.
A place to start ... start what? Start experimenting with the settings to see what does, and does not, work with your monitor.
I tend to start with the screen resolution. Once again right click on the desktop and click Properties, then Settings, and then drag the Screen Resolution slider a notch or two to the right to increase your display resolution. Now click Apply. It should change the resolution and display a small message box asking if you want to accept this setting. If you don't respond within some time, it assumes you can't see it, so it resets to the previous setting. The lesson here? If the screen goes blank or you can't read it, wait at least one minute before doing anything. The problem may resolve itself, and you'll be able to say "well, that resolution doesn't work", and move on to try another.
If, after a full minute or more the screen remains blank or unreadable, you'll have to reboot into safe mode, as we did originally, to restore your setting. (Why might this happen? Windows "remembers" and doesn't bother to ask you if you reset to something that you previously said worked. If you mistakenly said "this works" when it didn't ... perhaps by random keyboarding while the display was blank ... well, that could be how we got here in the first place.)
You can repeat this process of trial and error for each of the screen resolution, the color depth and the screen refresh rate. (For CRT displays, you really do want the refresh rate to be as high as your monitor supports.)


Method-2: If even 'Safe mode with command prompt' is not working try the following process:
1.Login using your user id and passwoerd. Wait atleaset 15 nminutes for windows to be ready. You can not see anything but you can use keyboard to execute the following:
2.Use "Windows+R" to goto Run.
3.Type "devmgmt.msc" (without quotes) and press Enter Key.
4.Press Alt+V and enter the aplhabet "v"
5.Press Alt+V and enter the aplhabet "w"
6.Press tab key and enter the aplhabet "v" (i had to do this 3 times)
7.Rightclick using keybord, press Down key and then Enter key
8.Press ALT+Y and wait for about 10 minutes.




Possible Solutions
Performing the following options (in the order they are listed) may help to ‘fix’ the problem.

Option 1:
1. Restart your computer using an anti-virus recovery boot disk, and perform a complete and thorough scan (including the MBR if you can) of your computer for viruses.
2. If a virus is discovered then this may very well be the cause of your problem. Unfortunately however, removing the virus may not actually fix the startup problem, because it has already corrupted the registry files.
Option 2:
3. Insert the Windows Vista installation disk, and restart the computer. This should automatically start the installation program, but you may need to press Enter during startup to confirm you want to boot from the installation disk.
4. Select your Language etc, and click on Next.
5. Near the bottom of the screen click on Repair your computer.
6. Select the operating system (if it isn't already selected), and click on Next.
7. Select Startup Repair and follow any instruction on the screen. Eventually you will need to restart the computer (don't forget to remove the installation disk first).
8. If the Startup Repair doesn't solve the problem, repeat all the step above and select System Restore, and follow the on-screen instructions. (This option will only be of use if a System Restore point was previously created while the computer was still functioning correctly).
9. If the System Restore option indicates that the disk has errors, click on the Check the disk for errors option. Note that this may possibly indicate that your hard disk may be getting bad and needs replacement, especially if similar problems are starting to occur more often.
Option 3:
Try this option if you have previously made a backup of your entire computer.
10. Repeat steps 1-4 of Option 2
11. Select Windows Complete PC Restore and follow the on-screen instructions.
Option 4:
Completely reinstall Windows Vista from scratch. Unfortunately this will mean losing all the files currently on your hard disk, but hopefully you're one of those wise people who have regularly created backups of their important files onto CD/DVD.
12. Repeat steps 1-3 of Option 2
13. Select Install, and follow the on-screen instructions.
14. Don't install any additional programs or allow any (automatic) updates to be downloaded and installed yet.
15. If after a few reboots the computer appears to be working fine, create a System Restore Point as follows:
• Click Start > Control Panel > System and Maintenance > System Protection > System Protection tab > Create
16. If the problem re-occurs before installing any other programs or updates then you probably have a bad hard disk, which will need replacing.
17. Download any updates for Windows Vista and then reboot your computer a few times.
18. If the problem re-occurs after downloading and installing updates, then probably one of the updates is corrupt. In this case repeat steps 1-4 and then 6 of Option 2, to restore your system to the Restore Point you created in step 4, and then don't allow any Windows updates until Microsoft has come up with a fix.
19. If the problem does not re-occur, then you can start installing other programs.