Reversed polarity on a Battery hook up, this could be a real PITA.

I would check all fuses (passenger side engine compartment, and the box under the dash inside the car).

If your not sure about fuses visually, then pull the fuse and check with an ohm meter.

Then check for set Codes and Reset if any found.

Your now in a position to see if any Codes set a new, and trouble shoot according to what you see.

$hit happens, so don't kill the Techy just yet, but I would find a new place to get my Car serviced...

*** You can also disconnect one lead of the battery, let the Car sit for about 10 minutes, then reconnect. If the stars are aligned and the Gods with you, maybe things will reset to normal and your home free...