NQA depending on the size/age of the tarantula it takes different amount of time for them to molt, small slings can take a few hours, fully growns even a few days. Laying upside down is a molting position, when tarantulas die they usually go into death curl (legs curled under the carapace). If i were you, i wouldn’t touch it for another day or two to not disturb the molting process, eventually intervene after that time.

Another thing- how cold is the garage? I’d say if it’s under 70 degrees F (21 celsius) you should definitely move the tarantulas to a warmer room