Server losing file shares

I have a Foxpro app crashing. I trapped the error to be "File in use
by another". This happens when the app tries to open a file on the
server. During the error trapping, I also trap the output of "net
files" on the server. The result of net files is:

There are no entries in the list.


This is impossible as there are dozens of users accessing hundreds of
files through the server shares. At any other time, I can issue a
"net files" and see hundreds of files in use.

What would cause this momentary lapse of open files on the server?