My husbands games alot. WOW, and some others. So his computer is a gaming computer. Cooler Master. It has been working fine for months. One day her started it up and nothing.....you can hear the drivers starting then no imput to monitor, mouse or keyboard. There is a dull light on the monitor, like asleep, but can't get any icons to click on. Also, F8 doesn't work either. Have replaced the battery in the motherboard, and not sure what to try next.
There is a lot of things that this could possibly be and it would be a matter of trail and error to find the route of the problem. As you have replaced the CMOS battery that would probably mean you cleared the CMOS too, so that would have been my first suggestion... unless you didn't have the power unplugged from the back?
Try removing the IDE, SATA cables from the motherboard to see whether the system comes to life, have had cables go faulty and prevent the system from booting before. But mostly it sounds either a CPU, Mainboard, Memory or Graphics Card problem.
Do you have another system? If so it may be best to try and check these things, with Graphics being the first, memory the second but other two would be a lot harder to diagnose.