Sleep, hibernate, restart.
Reboot is the process of restarting a running and loaded computer system. It can be both intentional and completely unexpected, when a Blue Screen of Death appears, a result of a crash or a malware attack. Such a Reboot unfreezes the system, restores the errorless state, often erasing all unsaved progress in text files, graphics programs, and other very important projects on which the computer user is working at the moment. All at the cost of fixing bugs and improving system-wide performance. Like electronic time travel, more effective but also more severe than the Ctrl+z combination.