I don't think there is an option to recover process instance if the underlying data base entry is gone. Engine simply won't find the process instance any longer so it does not exist from that point of view. It might still have some log entries but they point to not existing process instance info.
Something that might be worth trying is to recreate new process instance based on the trace data of the missing one and then make the new one to become old one. Tricky especially when it comes to process variables but maybe it will fit you case.