|It sounds like it might be the power supply that's the issue.....but it's very hard to tell without some hands-on tests. Your best bet will likely be to take it in to a reputable shop to have a professional diagnose the issue for you.|
Worst comes to worst, you could also get them to back up the data on that hard drive while it's there. But I suspect that won't be necessary as it's likely something that can be fixed fairly easily.
However, if it is something serious, like a blown motherboard, they would be able to do a backup for you I'm quite sure.