when I upgraded to Monterey my IMAP emails disappeared

IMAP mail servers keep the mail on there Servers until the user decides to delete them

Try logging into the Mail Servers Web Interface and see if the e-mails are still there.

Then check with the Mail Server for the required instructions to add the account to the computer.

Who is provider?

Since you said they were IMAP, try deleting the account and readding the account.