My five-year-old HP desktop running Windows XP SP3 wouldn't boot from the hard drive yesterday afternoon. I would see the boot sequence begin, then nothing but a black screen.

Fortunately, I had just done an Acronis backup to my external drive in the morning, and I have a bootable USB key. I put the USB key into the drive, powered up the machine, and put the USB key first in line in the boot sequence. Voila! My machine came alive.

But now I'm confused as to what the problem is and what to do next.

I assumed that my hard drive was toast. But now that the machine is alive I can see files on my C: drive that have changes I made just yesterday. Clearly the drive is not dead.

Here are my questions:

1. What could explain my inability to boot from the hard drive? What would a remedy be?
2. What's my best course of action? Should I replace the hard drive with a new one?
3. If I replace the hard drive, do I reinstall the OS and apply the backup I did yesterday?
4. If I decide that re-installing Windows XP makes no sense, how do I get back the Acronis backup that I did yesterday? I don't want to lose that.

UPDATE: I just learned one more key fact. I'm having some work done on my house. I neglected to shut my machine down before the contractor came. My wife said he shut down the power to do some work on a circuit and then powered the house back up. I have a surge protector, but is it possible that cycling the power did some damage?