on the screen before the login screen, where you hit space to bring up the box, click the power button in bottom right
while holding shift key down, click the restart option
this should load a blue menu
lets see if we can get into safe mode:
choose troubleshoot
choose advanced
choose start up options
hit the restart button
choose a safe mode (it doesn't matter which) by using number associated with it.
Pc will restart and load safe mode
simply getting into safe mode might fix problem, so you could restart and try
If it doesn't, go back into safe mode again and try this:
right click start button
choose powershell (admin)
type SFC /scannow and press enter
once its completed, type this command into same window:
Repair-WindowsImage -Online -RestoreHealth and press enter
SFC fixes system files, DISM cleans image files, re run SFC if it failed to fix all files and restart PC
if that doesn't help, go back into safe mode again and perhaps copy anything off PC you can't afford to lose. we might have to do a reset or something to fix it.