If this is 240V electric tankless, make sure unit is getting 240V.
Circuit can test for electricity if just one leg of 240V circuit is on, but the other leg is not.
http://waterheatertimer.org/240-v-water-heater-circuit.html#120-240All costs are energy.
Find a plumber trained in that specific brand of tankless.
Read manual completely several times, and make sure maintenance is done on yearly basis.
Read troubleshoot section completely.
Manual should have phone number of manufacturer.
Call manufacturer for name of trained service technician in your area.
Before installing tankless, make sure trained person is available in area.
This will give insurance that tankless can be successfully maintained, and it will help economy since tankless problems can go on, especially as unit ages.
Tankless should be de-limed each year.
Tankless cannot be exposed to more than 11 grains hardness in the water.
Water pressure, water flow, water quality, air quality, air supply, gas supply, venting, internal parts such as filters, washers, circuit board, dripping condensation are all causes of tankless failure and problems can be combination of several factors.
Problems can be simple or complicated.
Install tank type heater as back up for those days when tankless is not working.
http://waterheatertimer.org/How-to-install-gas-water-heater.htmlhttp://waterheatertimer.org/images/Basic-240-Volt-water-heater-circuit.jpghttp://waterheatertimer.org/Tempering-tank.htmlAdd a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.