how can i fix my motherboard that keeps shutting dawn?

If you have access to a used PSU, maybe from an older computer, try it.
You may not be able to connect everything, but enough to power up and know if it's a PSU problem or something more serious.

If you need to get another PSU, take a look at the various web stores for a new or used PSU similar to your actual one. New PSU are not that expensive.

Electro
April 2013

You may have to take your machine to the Computer Doctor - you don't say make, model or operating system.

It may well be that your motherboard is fine. It's possible that the motherboard is detecting a fault with the machine's power supply unit (PSU) and is forcing the machine shut down to prevent damage or loss of data.

Good news if this is the case - a PSU is not that expensive and if you ask the computer doctor to put in a donor (previously used) PSU you may get it half price. It's a ten-minute job to change a PSU.

Motherboard? May be better to buy a new machine. Sales have fallen 14% in the period Jan-Mar so prices are bound to get lower. Consider a new bottom-end laptop.

Good luck...

Peccavi
April 2013