Project not checked out to you anymore

[Jo]

A Project Manager, using Professional 2003, had opened a project file and was
working with it. A while later he tried to close the file (which also tries
to save it) and he got a dialog:

Project not checked out to you anymore.
The project is no longer checked out to you on the Project Server. To close
the project, click OK. To save the project as a file, click Cancel, and then
save it as a file on your computer.

What would cause a project to no longer be checked out to someone? (I
certainly haven't been checking in projects.)

 

[Jo]

Sorry about the double post. I had gotten an error, so assumed the first one
didn't go through...