No, you did not lose everything on your hard drive (most likely).
Its probably a hardware dying or some awkward setting.
First: can your computer boot up to the BIOS screen (the black background screen with like white or blue or low-resoultion text).
If so, good. If not, well your computer's motherboard might be dead.
After that: can you get to your BIOS settings or safemode (usually holding F2, F10, or F12 will bring you to your BIOS, F8 to safemode).
Can you get to the black background/Windows animation screen that normal starts up when your computer stars? If so, looking good. If not, then there is probably a bad file or something preventing Windows from starting up. At that point you will need someone who has the tools that can get the data off your HDD and back them up somewhere, then a Windows installation/reformat CD (that may or may not have come with your computer)