Your tachometer's engine speed sensor is mounted on the rear of the engine (Left Hand side) on the bell housing. It should be near the back of the oil filter housing.
The mechanic probably jarred something loose while working inside the dash. There is also a two pin connector that may have come loose. Find the engine speed sensor and follow the wire back to the firewall - make sure that connector is tight.
Here are is a thread...
Figured out why my Tach won't work