I am working on a friend's 21 inch iMac. He knocked it off his desk with his guitar while practicing one night recently. He said at first, it would not turn on. Later that week before bringing it to me, he tried it again, and it worked long enough for him to get a backup. He said after a few hours, it crashed to what seemed like no power (though as silent as the macs are, that would be hard to tell for sure without outputting it to another source - but he said the power button was unresponsive). At the very least, the screen wasn't working for sure. He said it would no power up for a couple of days, but once again, he tried it again and had it going for most of an evening. The next day it died mid-use once again. I now have it in my possession and was wondering what kind of diagnostics, if any, I may run to assess the issue. I have been playing movies and running random programs all day and nothing seems to want to make it replicate the issue. I was wondering if something may just be loose, or failing (like a bad power supply?), but I wouldn't know how to figure that out without opening it, and even then, I could only do so much. Obviously trying to avoid an Apple Store visit here. Any help would be greatly appreciated! Running 10.9.5 currently.