Unplug the PC. (Don't just turn it off. Completely disconnect it from the mains).
Unplug any USB devices. (If you had an external drive, such as a pen drive, plugged into a USB port it might be worth trying a normal start up at this point, rather than bothering with the stuff below straight away).
Wait a couple of minutes.
Plug the PC in and turn it on again.
During the boot-up process (i.e. before you get to a Windows screen), press F8. (You sometimes have to press it several times).
Choose the option to boot into Safe Mode. (Just use the simplest version, ignoring things like 'Safe Mode with networking). You'll need to use the Up/Down keys, and then 'Enter', to do so.
If you can get into Safe Mode, you might find that your screen looks rather odd. That's because the display driver hasn't been loaded. Don't worry, it's normal!
Simply restarting your computer (in the usual way, via the Start button) might be all that you need to do.
If you then still find that you can't get past the 'Starting Windows' screen, repeat the above steps to get back into Safe Mode and check the disk for errors:
https://support.microsoft.com/en-gb/help/2641432/check-your-hard-disk-for-errors-in-windows-7
If you can't get into Safe Mode at all, please post again.