Are you working with the same language pair?
Check your HDD for errs and then
1) Check Indexing service status (it often locks files for a 'quicker search')
2) Check file attributes and access rights (NTFS permissions)
3) Antivirus/ protection software can intercept/scan/lock files etc
Most likely, the TM is being shared through a network, or in two simultaneous Word sessions, or the previous translation session was not terminated properly. Do not service a TM currently used through network. If you're not networking, close Ms-Word. With your disc Explorer, find the folder where the translation memory is, and delete the translation memory file that has the ".net" extension - and only this file.
The glossary is a file too, so just duplicate/rename it whatever you want and re-assign.
Hope it helps. Cheers.