Sometimes if Word is interrupted it causes a failed write to a file - if this happens, then the temp file properties get stuck and it isn't deleted; this can cause it to list the file as 'active'. This causes Word to see the file as 'already open' and so means it won't allow you to open a second document in case changes have been made to the first one!

Close the document and Word and then go to the folder where the document is saved - see if there are any temp files there (make sure you change the view to allow the viewing of hidden files [Tools > Folder Options > Show hidden files and folders]). If yes - delete them, then try to open the document again.

Let me know if this works.