Start your PC, just as Windows attempts to load (spinning dots), press and hold Power Button for 5 - 10 seconds to perform a Hard Shut Down
Do this twice
On the third start Windows will boot into the Recovery Environment and from there you can access System Repair, Safe Mode, Command Prompt... etc.
Please try each option one at a time, then see if Windows will boot to a stable system.
Go to Troubleshoot - Advanced Option - Startup Repair
See if Windows startup can repair itself
Go to Troubleshoot - Advanced Option - System Restore
Check to see if you have a restore point you can go back to