Hi there and welcome
The ? Mark means it cannot access the hard drive.
I want you to first do the following please
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R. Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
Then tell me if you can now sign in