Hi there and welcome
Can you do the following please
At the login screen, hold down the SHIFT button and with it held down, click on the power icon and click restart
Still with shift held down, let it restart and it will take you to a menu
Choose from the menu Troubleshoot, then advance
Then choose System Restore
Follow the onscreen steps and preform a system restore, using the last restore point
Let this complete and when it has, tell me please if the computer then starts.