Thanks for reaching out for help. It's frustrating when these things happen but I'm sure we can get you going again.
Based on some of the other threads I've seen on issues with Hogfather, one thing you may want to check is to see if your PHP installation has the mbstring PHP extension and, if not, add it.
Another thing you could try is going into the user list file via FTP and manually editing your user account name to all lowercase and then try logging in with that (this is a temporary workaround as other users with cases in names would be affected too). The list is located in /conf/users.auth.php
Edit: Also what auth/login system are you using? Do you have any add-ons that might affect login? Because most Hogfather installs don't seem to be having this issue so we want to narrow down why you are.
Edit 2: This may also be possibly related to this bug: https://github.com/splitbrain/dokuwiki/issues/3113