Power it down - it's going nowhere. Press the power button for ten seconds then start it up again. If it doesn't offer you the StartUp Repair options menu, keep turning it off and back on until it does.
When you see that, select the Advanced box then Command and at the Prompt, type
and hit the Enter key. That will run Windows Checkdisk utility and the /r switch should make it find and fix disk errors. It's a long process but it's quite often the cause of what you have there. If a crucial file is sitting on a bad disk sector, it can't operate so fixing those errors might work for you.