Based on IRC context here, this was with ask.fedoraproject.org?

I don't see any way to delete you login credentials there.

Can you simply try and login again via facebook?

I didn't get any feedback from the user.

User provided following info:

"Credentials can be deleted from here:
https://ask.fedoraproject.org/en/account/signin/?next=/en/account/signin/
I deleted all credential of my old account (fedoraemon) but forgot to
create new ones, so I basically locked my self out of that account.
I tried then to add the same facebook credentials from a newly created
account but the two accounts didn't merge."

Right, can you ask them if they can create a Fedora account with the same email as in the original Ask account on https://admin.fedoraproject.org/accounts/ ?
If not, we can send them an email to verify their identity.

Please follow the steps in https://pagure.io/fedora-infrastructure/issue/5549#comment-40550 and let us know if you have any problems.