Check to see if your hard drive has multiple partitions.
It may be that the drive partition (most likely C:) with your Win10 OS is on is full. You may have a D:, E: or F: drive with the empty space.
A lot of programs will create their own folder or install to the C: drive by default.
If you download a lot of files it may be that that folder should be changed to another drive.
You may also want to check your recycle bin if you have deleted a lot of files.. They may still be sitting there in case you wanted to recover them.