Sounds like your power supply- in my unprofessional opinion. They are relatively cheap and easy to replace. My husband works in IT, and we have 11 computers (9 running, 5 on network), so I have picked up a little.
If it's the power supply, or even a motherboard falure or power surge fry, your harddrive will be fine. You can unplug it from your computer, and plug it into another one and pull off it just like before.
If it was a harddrive crash, it wouldn't have shut your computer off, so that's a good sign for your data.