The screen locker is broken and unlocking is not possible anymore. In order to unlock it, switch to a virtual terminal (e.g. Ctrl+Alt+F1), log in to your account and execute the command:
loginctl unlock-session 2
Then log out of the virtual session by pressing Ctrl+D, and switch back to the running session (Ctrl+Alt+F2). Should you have forgotten the instructions, you can get back to this screen by pressing Ctrl+Alt+F2