Firstly make sure that the cable attached from pc to monitor is securely attached and has not been pulled out. If that is not the problem this could be a graphics card problem.
Try switching off the pc and unplug from the mains with the pc shut down and unplugged depress the start button for 10 secs to remove any power still left in the machine.
Now start the pc and see if this corrects the problem. If still no picture depending on the make of pc it might be worthwhile removing the graphics card if that is possible and then reinserting sometimes if a pc is knocked this can happen. I hope this helps.
carlito corway
June 2008