Sorry to hear that, just went through this myself a month ago.
When you say "it won't do anything" does that mean it doesn't load properly or that you can't even tell you turned it on...does it beep, do fans spin?
When it booted, did it boot properly (without the mouse)?
First I'd blow it out with compressed air to make sure the board isn't shorting on a dust ball.
Try it with just the monitor, no mouse, and see if it boots. If so try the mouse. If that keeps it from starting...weird. Try another mouse.
If you can rule out the obvious stuff, get basic and try 1 stick of RAM and your hdd. If you can get it to post properly you know it's not the motherboard.
Good luck, hopefully someone else has some better advice.
In my case the mb died.