If it's a desktop PC power down and unplug your PC from the wall plug so it can't power up by accident. Take precautions against static discharge for the following. Touch metal first and don't do this on a carpeted area if you can help it. Do it on a table or wood floor. Now open the case and reseat all your add-in cards. If you don't want to actually remove them and reinstall them just press them down firmly to make sure they are fully seated in their slots. Then double check all your data and power cables to make sure they are firmly connected. Also check any cables for front panel USB ports or card readers etc. Once your satisfied that nothing has come loos, put the cover back on and check all the external cables. Then power back up and cross your fingers.