Does anyone know how to get back/ what mission to do to get Sanctuary 'back' , so I can place the AI in Moxxie's radio?
completed the Rising Action quest (and Sanctuary teleports away)
you will be unable to reach Sanctuary until
you've completed the Bright Lights, Flying City quest. The good news is that it's the very next quest in that chain, so you're not cut off for too long.
Some quest will have a status of "Blocked" and you won't be able to continue (or select them) at all until you find Sanctuary again.