How did you manage to delete your designators? If I try to delete them in my installation of AD17 nothing happens. Did you delete them in the library?
If you somehow managed to delete them on the PCB design simply go to Tools --> Update From PCB Libraries. Select all layers, click 'OK", and in the next dialog click "Update All" and "Accept Changes (Create ECO)". This will update all of the components on your board to match what's in the libraries, which should include designators.
I still don't see how you managed to delete the designators to begin with though.