There are a few steps that could resolve this problem
1) remove the power cable from the power supply unit and unplug it from the socket, leave the cable unplugged from both the PC and the mains supply for around an hour and then connect it up again and retry turning the PC off.
2) unplug the main 24 pin power connector from your motherboard and plug it back in again, do the same with the 4/8 pin CPU connector.
3) Clear the CMOS on your computer by either removing the battery and reseating it on your motherboard or by clearing it using the BIOS screen on your PC.
If none of the above techniques work:
1) replace the Kettle lead from your power supply
2) try the power supply in another system and if the problem still occurs replace the power supply
Thanks for reading and feel free to add any comments/other techniques you have.
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