Do a factory reset on the MMI. In the menu, select everything except the Sat Nav. There is a good chance since the car is 2012 it probably had a map update in its lifetime, so you could lose that. Everything else including phones linked to the car will be wiped.
If the garage sent out SD cards, then that’s dependent on the MMI staying on, and allowing you to update. If it does, then you’re going to need a trickle charger to the battery. Updating sat nav from SD cards can take a few hours.
if the MMI doesn’t stay on, then the car is going to need to be scanned with VCDS to see if there’s a power or critical error with the MMI. It will also show any other logged DTC errors.