Welcome Andy,
This does happen ocassionally, and is usually due to an install loop. I am afraid that the install has failed and will continue until you take some sort of action. It happened to me once, I was forced to shut down the computer and lost everything, including the OS. I was forced to use a saved image to reinstall my system. Usually the results of a forced shutdown are not as drastic, but may cause problems. I would try to;
Stop the install
if that does not work you may be forced to shut down the computer.
After this problem is resolved, please, download the free Macrium Reflect and back up your hard drive monthly or even weekly. In the case of problems, you always have a current back up.
The Importance of an Image of your Hard Drive

Check your hard drive, does it seem to be working or does it seemed to be stalled?
In addition, after this is resolved, you may also want to test your hard drive and make sure it is not failing.