One thing it could be, is one of your programs using more ram than you have. In that case your C-drive may have been used to store what didn’t fit into the ram. Some times you’ll get the space back once the data is no longer needed or when you close the program, but in my experience, I often have to restart the pc for all of the space to be free again. That also means, if you have already tried restarting, than I think we can rule this possibility out.