It's possible that damage to the motherboard was limited only to functions related to your screen. Depending on the level of damage, it's possible that something was holding on by a thread and the sheer act of opening the phone was the thing that made it not function.
However. It's also very possible that this fact is related to the work they did, especially since it sounds like it was working aside from the screen damage. I'm probably going to do some extra research about this phone specifically, but board level damage is not uncommon, even from reputable and certified techs. I have done things my own self and never had any idea until I looked into things further.