Help with Windows 10 restarting when I select shut down

I've got an older desktop PC that I still use. I used to run Win 7 on this machine and upgraded it to Win 10 a few months ago. Everything was fine until recently.

Now I've got a very obnoxious issue. When I select "shut down" the computer restarts instead. To get around this , I have to press (and hold down the power button) on the computer to force the power off. But now, even that method won't work. I am wondering if the computer is actually crashing each time I select "shut down"? But I don't see any error messages and the PC jumps back on the desk top very quick. But this is a problem I need to resolve. The only option I seem to have right now is to put the computer to sleep, unless, of course, I just unplug it from the wall! I've gone into settings, but I cannot see any options to fix this. The option is already checked for the PC to "shut down" when I select that option. Thanks for any replies here.

Install all important Windows 10 updates.

Open Power Options > Change what the power buttons do > Change settings that are currently unavailable > Disable Turn on fast start-up. This will disable Fast Startup.

Thanks for your reply. The problem seems to be resolved now. Appears the fast start up was the issue.

You are very welcome!