Need help with fuel gauge not working?

I'm not sure how it's set up on your car. But on my 70 dart, the fuel and temp gauges have studs that stick out of the back of the cluster and have some flimsy nuts that screw onto the studs and tighten up against the circuit board. They are used to hold the gauge in place AND to complete the circuit from the circuit board to the gauge. My gauges worked intermittently even after replacing the voltage regulator. My problem ended up being that the nuts had looosened up over time and slightly corroded. I snugged them up and backed them off a few times to clean the cointact area and it's worked perfectly ever since.

Your mechanic might have wiggled them loose while he was under the dash!