lost assigned members

[spurtje]

I had 11000 members assigned to one of the accounts of one client who is running these people.
Suddenly ALL assigned accounts before July 21 are gone. New once are added but the old ones are not assigned to the account anymore, although they are still in the system.
So the account that has these assigned members can see them as user, but cannot work with them.
We tried everything and have not the idea that we changed settings on that date.
Please help.
Irene