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This is too generic a question. The trouble shooting steps vary according to the severity of the boot problem and the version of windows.
A startup repair, Boot from install media, reinstallation are possible remedies. If you can handle advanced troubleshooting, boot console is very handy.
It could also be a hardware issue like hard disk failure.
The question needs more details but a startup repair should help. If you cant get to the screen with the start up itself then the windows boot files are of no use and you will have to fresh install the windows. If you have any important files on the hard disk I suggest you ask a professional to help you back them up.
Reasons not to run the Windows may be many - such as the problem with the HDD, Windows Registry ...My actions depend on many factors and messages you see on the screen ...
