You can either just schedule a new service for the customer from the View/Add screen, which automatically reactivates that customer, or you can go to the customer's profile->Actions->Reactivate Customer.
If you are on the Route->View/Add screen and you click on any time slot on the route->schedule customers->and go to schedule a customer that is inactive, you will get this pop-up window. If you schedule a job for this inactive customer, it will automatically re-activate the customer for you.
