You'll have to provide a bit more info on your system and what you have done so far to try and correct it. First, Was the system left on? If so, for how long? What did you do initially? When you say you "couldn't reboot it", are you saying it was on and didn't respond to your mouse and keyboard? Or was the system off and now won't boot? When you push the power button, what does it do?
See, lots of info needed and more.
Have you tried pressing the INS (insert) key to reset the BIOS during boot? Have you unplugged the Power supply for a few minutes, then trying to boot?