My webcam usb2.0 cant be detecteted

my webcam usb2.0 cant be detected.i was using it fine like 1 min ago and suddenly it cant be used.i try replugging it into all the usb ports but still the same thing can any1 help me?i tried to install the driver but suddenly it says 'please plug-in the webcam' and suddenly the webcam can be used and i installed it.when i restart the comp it cant be used. can any1 help me?