Windows has some very important files it stores on the root of your "C" drive (they are hidden so you never see them).
If these files are deleted, corrupt, or altered in some way then this can often stop Windows starting.
If, for example, you shut Windows down using the power off switch, or got a hard disk error, this could corrupt one of these files.
It is a chicken and egg situation, you cant get into Windows to fix them and you cant fix them till you get into Windows.
If you have a PROPER Windows disk (NOT a recovery disk) then this has a REPAIR option on it.
If you start the Windows install, and choose repair, it will replace the important Windows files with new copies and Windows may start.
Some others may suggest different things to try.
I hope you have your important data backed up !!