"This File is In Use..." but it's not

We are having some sort of network problem. A table in its own separate database file is stating that: (paraphrasing)

"This file is in use. Close the file and try again."

but the file is not in use. There is no locking file open, and furthermore, multiple users have used the file simultaneously for the last 12 years without a problem.

Of course the file is critical and everyone on the network needs the data but are locked out.

We cannot even do a Copy to move the data to another file name. I have never had a problem copying a database even when it was in use by someone else.

Some guru out there must have an answer.