Error message - "Cannot open company file ... "

I am trying to open one of our client's Sage 50 files, and I keep getting the error message: 

Cannot open company file ... ... Either another user has opened the company in single-user mode or another program is using the data. Please try again later.

No one else in the office has this file open, and I have gone into the Connection Manager and stopped and then again started the service.

Does anyone have any idea what is preventing me from opening the Sage 50 file?

Thanks in advance,

Kristine