Ihere isn't much you can "reapir", except replace one of the components that is not working.
Figuring out which one it is is the biggest task.

Only way to do it is the process of elimination:

Unplugg all the cards, drives and RAM sticks and try turning it on. If you dont get anything (beeps, post, video feedback) that would leave 3 possibilities: CPU, motherboard and PSU.

In case you make any progress, plug one thing at the time, and ONLY one at the time.

Btw, are there any beeps? Please post their sequence (Ex. bad VGA would make 1 long 7 short beeps)

yea, we need more information about the problem.. what specifically happens?

I have already dont worked you that it is the motherboard. So I was asking how do I diagnose the problem the motherboard has (i.e is it the bios, is it that the bios isnt getting power or is it something else. If you read post "760GXK8MC" you can read the full details of what happens. But I think it is the motherboard. So I want to know how to fix a motherboard or replace bios or whatever.

sorry I mean I have already worked out that it is the motherboard

Trying to repair the motherboard on a component level is a daunting task. Due to miniaturization there are few components on the motherboard that you can trouble shoot. If you look on the back of a modern motherboard you will see that most all of the components a surface mounted and very few components are though hole soldered. Most of the through hole devices are actually expansion slots or sockets. Even most of your ICs are surface mounted now, and even if they weren't how would you go about testing these? About the only component that you can check are the electrolytic capacitors, and most of the failed ones can be recognized by rounded tops, and may even leak at their bases. Other than the occasional diode that's about it for components. You can look for burnt traces and arc marks, usually if there's been a shorted connection there will be a visible smoke residue on the board.

If you are sure that the motherboard is the problem...replace it.

I cannot "see" anything wrong. I think it might be the bios can I replace this??

Yes you can. That and CMOS battery are only things that one can replace on motherboard.
BIOS replacement is needed ONLY after a bad flash. No need to go and buy a new BIOS if the motherboard is fried. So, did you flash the BIOS before it stopped working? There is a way to recover without buying another BIOS chip. All you need is floppy drive, bootable floppy disk and BIOS file/flashing program.

you have not told us what happened. Only that you "know" it is a motherboard problem. The only advise anyone can give to you is to go and buy another motherboard.

This wouldn't be that foxconn board that you bought at Ebay would it????