To save having to use the mains switch.
Press the power on button on the front of the PC - and hold it pressed - it should take 4-6 seconds for the PC to turn off.
Then it should turn on again as normal.
There are lots of non-catastrophic problems which can stop the PC turning off when it closes down - normally a software problem. It may take a bit of time to find out exactly what, but if this is the only issue don't worry whilst we and you are looking for the reason.