Hey. well me bro has this pc and it wont start up. Everything starts normal but no beep when the computer tries to boot up. i have taken his cmos battery out and put it back in, but nothing. i tried the battery in my pc and it worked. Then i put my battery into me bro's pc then it was working. Now after a week it aint working. non of the cmos batteries are working in it now but they work in mine.

If it works in your computer, then it works. His motherboard is probably fried, but it's hard to just guess when you don't know any of the other symptoms. Could also be the power supply. Open the case up and try and turn it on. What happens?

I think it is the motherboard to be honest. i have changed, everything i possibly could, including power supply. When it starts up the screen stays blank but the fans and everything carry on like normal. also you have to turn the pc of from the back because it wont turn off from the button on the front

Why don't you try clearing the bios and see what happens. Even though what you've said so far would point to something with the mobo, I'd still suggest you try it. Should just be a jumper, read ur mobo instruction manual for exact location and procedure