Unable to unlock system preferences

To expand on HWTech's great advice...

See if you can create a new administrator account by restarting the Setup Assistant:

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDone then ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Be very careful to notice the spaces in those Terminal Commands.

Once you've done that the computer reboots and it's like the first time you used the machine, except all your data will still be there. Your old accounts are all safe. From there you just change all other account passwords in the account preferences!!

If you changed the user account short name or any account settings related to this, then you did so incorrectly and should have followed the instructions in this Apple article:

https://support.apple.com/en-us/HT201548

To fix the issue requires creating a new admin user account and following the instructions in the article I just linked above. I've posted instructions for doing creating a new admin user account multiple times on these forums so do a search to find the instructions.

Your other option is to restore from a backup made before you made these changes.