Starting from best way to worst way to reboot a computer
Start Menu | Shutdown | Choose either Shutdown or Restart. Wait for the computer to at least get to the shutdown stage.
Press the power button quickly, once. This is just like asking the computer to shut down from the start menu.
Patience is your best friend, waiting (5-10mins) will minimize the likelyhood of having further/worse problems.
If the computer is responding slowly to the above shutdown commands most likely one or many things are happening:
Individual program (or part of your operating system) is using 100% CPU (usually a bug/problem with what it's trying to do). Use the Task Manager to diagnose if this is the issue
Computer has encountered a bad sector on your Hard Drive (HDD) and it's trying multiple times to read/write data on your Hard Drive. With patience and luck, it will eventually be able to read the data it's trying to, then the HDD will stop using that bad part of the disk automatically. If this is occurring a surface scan of your Hard Drive is necessary (will take 30-300mins and you will not be able to use the computer) chkdsk c: /r
Using these techniques in order to force a reboot to a stuck/hung computer. Only use these if you've already tried the safe method, AND WAITED 5-10mins!! You are risking data loss/non-functioning computer by using these techniques:
Press the hardware reset button on the computer
Press and hold the power button until the computer force turns off (this always works!). Usually take 5-10 seconds of holding the power button down.
NEVER DO THIS!!
The worst thing you can do to a computer is physically unplugging the power cable from the back of the computer. There is a ground wire (the third plug) that is designed to take excess power that is in your computer and safely dispose of that power. When you unplug a computer you are removing it's ability to safely dispose of that excess power, and as a result it will usually go and affect the hard drive. Usually destroying either data, or causing the computer to be unable to boot.
This is the number one cause of non-booting computers after a power outage/forced power off.