There are many reasons why this might happen so you might have to try to identify the cause by a process of elimination - made more difficult if the problem is intermittent.
Make sure all drivers are up to date and scan for viruses.
Ask the user to note down when it does and doesn't happen and to track whether they use any specific programs or do anything else when it does.
Close down programs from within Task Manager
just before each time they shutdown and note whether the issue occurs.
See if anything untoward was logged in Event Viewer
Try booting/shutting down from Safe Mode
several times. Does the problem reoccur? If so the issue is likely to be basic hardware, OS/Driver related. If not then it's probably down to some user software.
to check the drives for damage.
Use System File Checker
to restore any damaged/changed system files.
Take a look here
to learn how to disable programs from starting up automatically upon boot. By disabling one of/all but one of them at a time, shutting down/rebooting a few times, you might be able to find the culprit.
Take a look HERE
for some more specific reasons this might happen.
Hope it helps.
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There are only two men I trust with my computer. One's me. The other isn't you.