Ignore the issue and continue attempting to log in repeatedly.

If you can't remember your CID, go to my.vatsim.net/forgotten-cid

The key is remembering that there is a ton of helpful content at my.vatsim.net

Worst case, you can open a Membership support ticket at the VATSIM Support Center

Create a new VATSIM account with a different email address.

Ask a friend for their VATSIM CID and try to use that to log in.