Can you help me?

[JenMacca]

I don't think it's something as simple as a screw being loose, because the last time this computer got worked on was last January, and I'd imagine if it'd been loose all this time, it'd have been giving me those beeps for a long time now. So you're pretty sure it's the motherboard then? Is there some type of diagnostic test I can do on here to find out if it's corrupted or what without actually taking the computer apart? What sort of things would mess up a motherboard, anyway? (I really do apologize for all the questions, I'm pretty sure this is going to be the first time I have to work on the computer myself to fix it, so I want to find out all I can before doing anything big) And about how much do motherboards cost, are they really expensive? *hopes they're not*