Revert Server to old data state

i had a problem with my server HW and have to revert the server to the backup, which was created two weeks ago. The clients inbetween synchonised new data and have a newer state locally, but the server data is gone.

What happens, if i revert the server to the old backup state and a client with a newer local data/database state connects?
Will new new data, synct to the lost server state, resynchronised somehow?

BR, Cornelius