It's almost unheard of for an automatic gearbox to fail without any previous problems
If it hasn't had any previously then I'd be checking first that the cables are moving the selector on the underside of the gearbox. If they've come adrift it won't select any gear.
If they're intact I'd be checking the fuse for the Autobox ECU which is in the underbonnet fusebox.
After that it would be a wiring check
Wouldn't trust what the garage says.
Russ