Seems like a setting issue. Maybe Windows is set to use the "projector" (second screen) only. Then the first screen turns black. Try Winkey+P, 3 times arrow left and enter to change from second screen to primary screen.
But it could be some driver issue also, if you changed something in the configuration or hardware. If you didn't that's unlikely
And it could be any issue with your account. Do you have an autologon configured, or do you need to type something in the welcome screen to login and can't do that because it's black. Then that's unlikely.

See what happens if you boot in Safe Mode. Things to check and try there:
- screen settings
- driver (in Device manager)
- sfc /scannow
- system restore back to when it worked
- a new Windows account