There have been an increasing number of computers lately with hard drives mysteriously filling up over just a few days or even hours, and it is most often due to a glitch in some windows 10 1903 builds that causes the os to generate tens of thousands of .evtx files in C:\Windows\temp for reasons currently unknown – it seems to be connected to the Windows Store though.
You can quickly delete these by opening cmd and running
del /s C:\windows\temp\*evtx*
del /s C:\windows\temp\*appx*
If the hard drive truly does not have a single byte of space left, you probably can’t log in normally. Use a Backstage session since it gets you straight into an elevated cmd.
After clearing these, the computer must be updated to Windows 10 1909 or higher to prevent the problem from reoccurring.