Serial Monitor will not open

The serial monitor has been working fine. Then I get the following message:
UserSideException: could not open port ‘COM20’: PermissionError(13, ‘Access is denied.’, None, 5)
I searched and saw that this issue has been raised previously but without any definitive reason why or solution. I tried changing the COM port in Device Manager from 9 to 20 but that did not help. I got the same error with 20 instead of 9.
Is this a PI error? If so, how is it corrected.
Thank you,

So I am the only person in the world who has had this problem? Ok…fair enough