The first thing you should do is to try and make sure everything is seated properly. Make sure the graphics card is in the motherboard securely, the RAM, etc. It could be that when you tripped over the cable that something was pulled/shaken loose. If a component is in, but not secure, then it could shake slightly and work then after a hard stomp beside the PC get shaken again and stop functioning. That is your best chance of fixing it cheaply and quickly.
After you do that, if the problem persists, you could try downloading a BIOS update and installing from inside the BIOS, as it could be an issue with the motherboard and not related to the HDMI cable getting pulled. The motherboard could also have been damaged, however, and ultimately...