Yes, sorry for not adding anything. According to Apple's document, the username must March the name of the home folder. You only changed the username without changing the name of the home folder, so you’ve hosed that account.
You will need to create a new administrator account before you can fix things. Follow the instructions at this link to create a new admin account:
If you can’t remember your Mac's login, you can create a new administrator account by using the single user mode of your Mac.
Once you have that done, you should rename the home folder of your original account to match the new account short name. Hopefully the account will work after that. If not, I would recommend restarting to recovery mode and restoring the computer from your most recent backup.