PC stopped working

[CaliforniaCowboy]

I wouldn't waste time with the motherboard battery unless you have one on hand to try (CR2032). All the mobo battery does is hold the clock settings, you should still be able to boot with a dead one.

[tenemae]

Does it beep at all when you turn it on? Most older motherboards have beep codes to tell you what is wrong. If the motherboard isn't throwing an error code, it's probably the motherboard itself.

[WoodTurner]

Could be a failed ram stick as well.
If it has two ram sticks, try pulling one. If that doesn't work, put the one back in and pull the other.

[the86d]

I am guessing OP is not running a UPS?