I had many many problems before with my computer. Most of them as I found out was due to lots and lots of bad sectors. Eventually I had to install a new HDD but there's a problem still exists which is.. the computer won't hibernate or hybrid sleep.
After a long while it shuts down by itself - I don't cut the power off or anything like that - but the next time I turn it on I get the "Start windows normally" screen. Same goes for hybrid sleep, but I'm new to that since I recently converted to Win7 recently.
I used to have this problem too when I was using WinXP (last period though, not the whole time) but with a slight deference: the computer used to get stuck on the blue screen and never shuts down. Therefore I'd have to cut off the power myself.