Dell Inspiron 1564-5161 black screen-ventilator remains on.

first stage : after normal startup and normal function, suddenly very blurry image, sometimes combined with vertical white / blue bars on the screen . A few seconds later I got a black screen ; no error messages appeared, image via VGA cable to another screen was also black , fan continued ( hard) turning, the sound of e.g. an online radio went off at the moment the screen becomes black.
Shut down of the laptop is only possible by pressing the power button for several seconds.
Reboot was sometimes not possible, sometimes the laptop started for only a few seconds and sometimes the laptop functioned for hours without any problem .
After a few days , reboot was no longer possible ; the two outer LEDs (power and battery) went on , the LED in the middle lite blinked, the fan was running, a recovery CD in the tray turned , but the image remained black, no peep sounds was heard ( except 2 times if the power was not completely plugged in) . Turning off the laptop was then possible with a short press on the power button . Startup without battery didn't change anything (except for extinguishing of the LED) .
Because the laptop became warm , I cleaned the fan and the cooling , the entry of the fan was found obstructed by a layer of dust .
After the cleaning of the fan , the fan runs smooth and (sometimes) fast and the laptop gets no longer hot. However the problem with the black screen is not solved . I hope someone can help me thanks inadvance (sorry for the bad english)