Can it be fixed by replacing the tachometer? Thanks

 

Replacing just the tachometer won't fix it.

The odometer and speedometer drive circuits receive pulses from the PCM, which gets it signal from the countershaft speed sensor on the transmission.

Without testing; problem may be with wiring, connectors, countershaft speed sensor, PCM, or gauge assembly.

 

Just replaced spark plugs, valve cover gasket and did a transmission fluid drain and refill. Could it be that the mechanic disconnected something by mistake? Thanks

 

Did the odometer and speedometer stop working after this was done?

If this happened right after those things were done, check to see if the wiring and connector to the counter shaft speed sensor is okay. But, there should have been no need to even touch that connector or sensor.

 

Gauge assemblies are usually pretty solid, they don't fail often. Your problem is most likely the "VSS" or Vehicle Speed Sensor (IF MANUAL TRANSMISSION) - OR - the "Transmission Speed Sensor" (IF AUTOMATIC).
They average $50 or so depending and are pretty straightforward to change. You could do a bunch of diagnosing or you could just buy the sensor and try it.

 

6th gen Accord V6 didn't come with manual transmission; so, no VSS on a V6.