When a practice is deactivated, all open and closed claims are set to a "Discontinued" state. They cannot be re-opened just by reactivating the account. A user will have to manually resubmit them.
Otherwise, the account and claims can still be worked. Notes can still be taken, but the Work Center > Work Claims list will not show claims because they are no longer considered "Active".
Users can access the deactivated practice by changed the filter to "All" or "Discontinued Only" when looking in Managing Org Admin or Org Admin.