My 2009 GMC Sierra heater will not put out hot air.

It's certainly possible that a connector was missed , they concentrate on the body work not anything mechanical . But something may have gone wrong by pure coincidence , Check the coolant level and heater hoses they should both be hot , I think those trucks have a blend door that dictates heat and are not that accessible , that's what I think

With that much damage, the truck must have taken a big jolt; that could have jarred something loose. Could be a Mode Door actuator or its connection; if stuck on cold, that'll close off heat. Or it might be in the climate controls. Have a mechanic check it out; should be added to the insurance collision settlement.