Inactive Apple ID

I changed my Apple ID without first signing out of the previous one…I did not know you had to do that. Everything worked fine for a while, but now I get a request to enter the password for the old Apple ID on my iMac. I do to remember the old Apple ID password. If I try to retrieved it or change it, the system tells me that that Apple ID is inactive, so cant change the password. I am stuck. I seem to remember a while;e back having this same issue and it was fixed by deleting some .pst (I think) file from "Library". I could be wrong here, but that is my recollection.

Any help with resolving this will be appreciated. In short, how do I delete a prompt to enter my Apple ID password on an iMac when the Apple ID being asked for is inactive and unable to retrieve/change its password. Thank you