Do all the lights that normally come on when turning your computer on turn on or does it seems dead, and do actually hear the hard drive spinning up if you do try plugging an external monitor into your VGA or DVI port and using the display switch key on your keyboard. If it works then a connector in your monitor either came loose or the monitor is shot and needs to be replaced. If this doesn't work check the manual to figure out how to get to the memory bay, once there check to see if the memory sticks came loose like Saaiello had said. If they are pull them out and reinsert them per your User's manual instructions.
Let us know how it works out.