I am not a Windows expert but let's do this. Let's try a Windows 10 repair.
To repair a current Windows installation, you need a bootable USB.
If you have another PC:
Download the following tool:
Download tool now (microsoft.com)
Insert a blank USB with at least 4 to 8 GB of space in it.
Run the tool and choose Create an installation medium for another PC.
Select the correct USB.
Let the process finish. Make sure you don't turn off the PC or remove the USB.
When done:
Insert the USB into the affected PC.
Turn on the PC.
Immediately, hold down the boto key of your PC. It's mostly F12, F10, Delete or some special key. You can search for it on the internet.
Select the USB drive and press Enter to boot from it.
In the Windows 10 installation environment:
Click on Next.
Click on Repair this computer and wait for some time.
Click on Troubleshoot > Advanced Options > Startup repair.
Let the repair finish. Do not interrupt it.
If the startup repair failed:
Go to Troubleshoot > Advanced Options > System Restore (this will work only if you have a system restore point)
Select the latest point and restore from it.