My mac isn't booting after trackpad was refixed

This gets into when you gave the system to them to be repaired. Was it still under warranty at that time? Do you have paperwork which clearly expresses that? Then they do owe you a repair under the repair warranty, but time is ticking!

If it was not under warranty, was the tech trying to do you a favor seeing if it was a simple issue? Did he take it to the back for a few minutes and then come back with the bad news?

Now the tricky part! Did you have an accident? Did you spill a drink or get the system wet? If you did the internal markers could be the reason the tech didn’t want to fix your system. So its possible your system just couldn’t survive the damage

So now can you get your system fixed?? You could try to fix it your self, seeing if replacing the trackpad is all it needed MacBook Air 13" Retina Display 2019 Trackpad Replacement

These are not easy systems to fix, not engineered to be easily repairable. So be prepared it might be a lost cause! Sorry ;-{