Don't freak out yet as the data can usually be recovered if it's a bad HD.
I know this sounds odd but some folks have had success by removing the HD, placing it in a zip lock baggie, then placing it in the freezer for a couple hours. Then remove it from the freezer, plug in the cables and try booting it. If it works I'd suggest immediately doing a drive copy to a new HD as soon as you can get a replacement HD.
Obviously if you find your copy of Windoze you could try a "repair" but if the HD is failing this will not correct the problem for long if at all.