Computer starting, no beeps, no signal.

hey. first off, this is a pc i've had for quite a while... haven't had a single problem for as long as i can remember. upgraded video card yesterday, and was working fine. case was a bit small so it was getting hot so i turned up the fans which solved the problem. only thing is that 2 of the usb ports on the back of my computer stopped working. so i went to work, and when i came home i decided to try and fix it through the BIOS. i was looking around at all the settings and i accidentally went into the O.C. option. the computer restarted and would not load a signal but the keyboard still had power, then it restarted again and nothing. since then i've had dramas... tried absolutely everything i can think of. the computer starts up fine, but get no signal. no beeps, nothing. and the VGA light comes on the motherboard. can't be a problem with the card coz i tried my old card as well and the same thing happens. tried the jumpers, removing the mobo battery.. taking out the card and using another one, different monitor, unplugging everything except for boot drive.. tried onboard graphics, removing memory.. and i get the error beeping that i have no memory connected. but all the while still, no signal and the VGA light is on. it's frustrating the hell out of me because nothing is working and i have no idea what the problem is.

i have an asus p8z68-v pro mobo
intel i7-2600k cpu
gigabyte gtx 670 gpu
2x 4GB g.skill ripjaw RAM..

all working fine up until this.. i assumed it had been auto overclocked too much but no kind of reset seems to work. any advice would be greatly appreciated !!