Computer: Unknown Type, possibly an office computer.
256 MB of RAM
other specs unknown.
OS: Windows XP

Yesterday, while online, my computer mysteriously shut down. It STARTED back up, but strangely, it SKIPPED the usual setup/bootup screen, and went straight to the Windows loading page. It stopped when the screen went blue and said "WELCOME". I restarted the computer a few times, and eventually it went to the desktop. All of my files (pictures, music, even the regular games that come with Windows), including shortcuts, even my wallpaper was gone, excluding the recycle bin. I searched the comp, and nothing turned up for any file, and when I went to defragmentation, and I had 20% more free space than normal. I went to the local C: drive, and saw that all of the files that I had on my computer were still in there. I checked, and they all seemed intact and normal. I even tested it by loading up a game, and my saved data from it was still on the computer, proving the memory was still the same.

My father experimented with the hardware a bit, changing the hard drive with another and back and re-inserting some other parts, although it didn't seem to change anything.

I believe it could be a hard drive failure as my father believes, or a possible BIOS's problem as I read from the FAQ's, but I cannot be sure.I suggest you run a Checkdisk on the hard drive to check for any problems.
On your desktop select Start and Run...
type cmd and hit enter in the run window.
In the cmd window type chkdsk c: /R and hit enter. It will ask you whether you WANT to check the partition the next time the system restarts. Press Y and hit enter.
When you restart the computer chkdsk will run. Depending on the size of the drive this might take a while. So do it when you don't need to use the computer.
If you got more than one drive I suggest you do this for them all. Just replace c: with the drive letter. (Only do it on hard drives, not cd/dvd drives.)