Your original description in your first post fits exactly what happened on my friends computer a few months ago
--- The computer needed to be reverted back to Windows 7 via Advanced Startup options and solution in her case is in the windows10 forum link in usasma's post #3
I can review my notes and would like to submit some ideas but I need to know the following:
"We are going to try reinstall her OS with Win7"
--- How can you do that if "The other day she couldnt start her computer"
Is the computer still in your hands or has been brought to a specialist?
Is the computer a laptop or a desktop?
Has the computer been running Windows 10 for a while before the problems you let us know about started to happen?