What kind of Computer is it? Same thing happened to me and mine is a Dell Inspiron 1525. But, I hope you did weekly backups (or at least one for that matter) if so, you might be able to save your computer by going into safe mode, and going to system restore (or repair, try both) and doing that from a back up point. Other wise, your mother board and hard drive might be shot. How old is the computer? I had a 1 year warranty, so I got fixes for free. I had to replace my mother board, hard drive, screen components, basically 70% of the computer. Took Geek squad a month and a half to fix, they sent it to Kentucky because they suck 0.o
Don't try to format it your self if you want to. Get computer people to do it, it will save you a lot of trouble if you can't fix it.