why does it say that I have used 38.9 GB but I have no files

Have you cleaned your trash as well (which is counted as well)? if your trash is also clean, this data is coming from cache which happens some times. You can raise a support ticket and they should recalculate the same for you.

This is documented here as well: https://support.box.com/hc/en-us/articles/360043695334-Storage-Usage-Or-File-Count-In-My-Account-Is-Incorrect