Computer wont detect monitor

Hi, I just recently fixed my friend computer (turned out to be defect RAM) I took out the ram from my computer to check with his. When i pluged back into mine, i pluged it in reverse so the computer wouldnt boot, i took out the motherboard battery (thinking maybe a reset would work...) and put it back, only to find out it was the ram, so I put the ram back into place, put back the battery, but now my computer doesnt detect the screen anymore! I dont know what to do help.

board: ASUS P7P55D-E LX
videocard: HD6850

Thanks!