Locked, read-only, non responding file problem

A file I have been editing suddenly gives the following when attempting to open it...

"'filename.xls' cannot be accessed. The file may be read-only, or you may be trying to access a read-only location. Or, the server the document is stored on may not be responding."

Great use of grammar!

I thought the file may be locked so I made a copy but the copy of the file gives the same results.

Any idea's out there?

Tom