Hey there, Bigfatmeany.
Judging only by this description I can't really guess what happened with the rest of the storage capacity. If you think you might have drive issues, you should backup your important data, just to be on the safe side and try to figure it out afterwards. So after you've backed up everything, I'd recommend that you take a look at this article: http://www.howtogeek.com/125923/7-ways-to-free-up-hard-disk-space-on-windows/. When you scroll down, there are a couple of options, which might be able to help you figure out what's taking up all your space.
Also, would you please be so kind as to post a screenshot of what you see about your drive and its partition in Disk Management.
Hope that helps. Please let me know how...