It's possible that your Master Boot Record is corrupted, so the computer knows there's a hard disk there but doesn't know to boot from it.
You can run Microsoft's own standard utility to repair this. Since you mention MS-DOS in your question, I'll assume you've booted from some sort of boot disk. At the command prompt, type "FDISK.EXE /MBR" including the space but not including the quote marks. This should rewrite the master boot record and let the PC start up again.
I don't think this causes any data loss, but if you're nervous (and I probably would be!), you could put the disk in another computer and copy all the important stuff off it first. It may not be bootable, but it probably is readable.