I have a ta2000 vac heat pump thermostat
, nothing works everything was working fine but now nothing works ,no heat ,no air ,not even the lights on the thermostat,where should I start troubleshooting

Model number and brand of outside heat pump unit.
Model number and brand of the inside unit.
The model number you posted for the thermostat is incorrect try the brand and model number of the thermostat again.

Check the breakers to the indoor and outdoor unit.