i would check everything has been plugged in again first, bodyshops are notorious for missing small details like that out, i see it on a daily basis! or a trapped wire where it hasnt been routed correctly.
not saying its 100% that, but you need to rule it out first. have a quick visual under the seats too, see if the occupancy detector has been plugged back in.
what fault code is stored, does it come up with a specfic airbag, or side?