Computer has been shutdown but the fan won’t stop

Hello everyone,
I shutdown my computer after using it but the fan won't stop. The monitor is already turned off but the fan is still working. I waited for few minutes thinking that it will stop but it didn't. This happened twice already. I need to turn off the computer voltage regulator to be able to stop the fan. I'm just wondering why it's happening? Please help me determine what the problem is. Thank you so much.
Anne
