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Hi there Colin and welcome
Can you power off the computer then hold the left SHIFT key on the computer then turn it on and keep tapping F8 until you get to a menu.
If you do not get to this menu, try a few times as it can be tricky, then choose from the menu Troubleshoot, then advance then System Restore.
Follow the onscreen steps to perform a restore from the last restore point.
Let this complete and when it has, tell me please if windows will now start.
Let me know the results please.