Did you open the mail merge database file in Access? If so, the problem is
that /Word/ only lets you edit Access database files that it thinks are
"Office Address Lists" (OALs). When Word creates them, these have a
particular database structure, with one table with a specific name, and one
query with a specific name. If you add more tables or queries or change the
database structure in other ways, Word will no longer treat the .mdb as an
OAL and will not allow you to edit it. Unfortunately,
a. if you edit the database directly in Access, Access may change the
structure even though you may be unaware of it - especially if you do
anything like create a report or an editing form.
b. I don't know how to turn that database back into something that Word
recognises as an OAL. All I can suggest is that if you did do anything in
Access that obviously created a new structure such as a report, a form, a
new table or query, you delete those in Access and see if that does the
trick.

Peter Jamieson