According to the official Windows support site, a reason why a computer running Windows will not shut down may be that a device driver or program is interfering with the standard shutdown process. Closing down all programs manually may solve the problem. Finding the latest drivers for a device in the computer might also fix the issue.
Other problems that can halt the shutdown process include viruses, spyware, malware and similar malicious processes that are accumulated through use of the Internet. In instances like these, drastic measures may be required to force a shutdown. Such measures include a hard reset or utilizing Task Manager to find the offending processes to end them directly. A hard reset requires pressing and holding the power button until the computer shuts off completely, skipping the soft reset function entirely. It is not recommended to do this, as it could damage the computer.
In the event that malicious programs interfere with a computer's shutdown process or other functions, it is a good idea to find and utilize proper anti-virus software to defeat the issue. Once the computer is clean, it may be able to do a soft reset without further problems.
A computer failing to shut down may also be a hardware issue requiring the expertise of a computer technician to get to the root of the problem. The power button could break, or some more serious issue may be preventing the proper shutdown process inside the computer, putting it at risk for future damage.