First check for bad &bulged caps(capacitors) or leaky ones ,using a ohm meter(if measuring the cap, the needle drops,it's leaking). Second change 3v battery after cmos reset.(sometimes you need to let board sit for a night with shorted out -/+ and no coin battery in.) 3v coin battery life is about 2-3 years in normal use. Some boards don't like 2.9v & under drop.
I salvaged a few boards using above procedures. If still no boot ,check to see if it wants to read floppy or CD after -power-on. It can sometimes be a corrupted bios ,you can fix from disk or cd.
However, listen for bios beeps, it can better determine a issue. If all fails ,maybe a vrm or chip is blown/ short circuited.(more difficult to determine or fix)