Log into your server with root privileges. Then start a Ruby on Rails console.
Start the console with this command:
gitlab-rails console production
Wait until the console has loaded.
There are multiple ways to find your user. You can search for email or username.
user = User.where(id: 1).first
user = User.find_by(email: 'firstname.lastname@example.org')
Unlock the user:
Exit the console, the user should now be able to log in again.