Try this:
Power down, all the way off.
(push the power on button and keep holding it down for about 5-6 seconds, the display should "go dark")
Now, try INTERNET recovery:
Press AND HOLD these keys:
Command+option+R
KEEP HOLDING DOWN THOSE KEYS, and press and RELEASE the power on button.
DON'T LET GO OF THOSE KEYS (all shouting is intentional).
When you are asked for your wifi password, NOW let go of the keys and enter your wifi and password.
Now the internet utilities should start to load ("as the globe spins").
Be patient, it takes a while.
Do you get to the utilities screen?
If you get this far, open disk utility.
IMPORTANT STEP:
Go to the "View" menu and choose "show all devices".
Now, look at "the list on the left".
The "topmost" item should be the physical drive inside.
Can you see it?
If so, click on it to select it, then click "erase".
Choose "APFS, GUID partition format".
The erase shouldn't take long.
When done, quit disk utility and open the OS installer.
Start clicking through.
If you have a backup, connect it now.
When startup assistant asks if you wish to migrate from another drive, "point the way" to the backup and let SA "digest everything".
SA will present you with a list of stuff to migrate.
I suggest you just migrate it all and let SA do its job.
When done, you should see your old login screen.
IF YOU DON'T HAVE A BACKUP...
... just create a new account when prompted, and finish setting up that way.
Good luck.