Here's a good online resource guide -> http://fixingmycomputer.com/ start with the 'Flow Charts'. There's a few online 'Books' here's one ->
http://www.freeopenbook.com/upgrading-repairing-pc/main.html
The most common problems with MOBOs:
1. Corrupted BIOS {general corruption, bad flash, bad CMOS battrery}
2. Shorting {HSF backplate, standoffs, peripherals including case headers}
3. Burned-out MOSFET, capacitors, etc.
4. Generalized physical damage {micro-cracking, broken connections, debris/corrosion}