PPC Doc wrote:

In this case, "group" means multiple individuals listed as one
contact. In other words, a distribution list.

DLs don't disappear on their own, any more than do other Contacts folder
entries. One mistake a lot of people make is that when editing a DL and
they want to delete a member, instead of clicking Remove, then click the
Delet button (X) on the toolbar, which deletes the entire DL not just the
member. However, I would think you'd remember doing that.

Unless you can recall what was happening at the time when they
"disappeared", no on will be able to explain it. The only advice I can
offer now is, "make backups frequently."
--
Brian Tillman