How to determine what the repair people did to fix my MacBook?
I work at an Apple Authorized Service Provider, and the fact that they won't tell you what they did means it was probably just an SMC reset that took two seconds. From Apple Support:


  The System Management Controller (SMC) is responsible for many
  low-level functions on Intel-based Macs. These functions include:
  
  
  Responding to presses of the power button
  


I'm guessing they didn't tell you this because all it takes to reset the SMC is holding down 3 keys and the power button at the same time while plugged in, and you'd possibly argue about having to pay them. It seems pretty shady of them, because we have machines come in needing a simple SMC reset all the time, and most of the time we waive labor charges, but for them to not even tell you that that's what they did doesn't make a lot of sense.