There may be a read lock on the file that you are attempting to open. Another user may have the file open, or another application ...

There may be a read lock on the file that you are attempting to open. Another user may have the file open, or another application that has a link to the file may have placed an exclusive lock on the file, preventing Word from opening the file. If a custom application has opened the file, it may have opened the file using an incorrect method.