The user manual for your unit can be found here. According to the document the following are possible when no codes are displayed:
Clean inlet water supply filter.
On new installations ensure hot and
cold water lines are not reversed.
Check for bleed over. Isolate unit from building by turning off cold water line to building. Isolate the circulating system if present. Open your pressure relief valve; if water is flowing, there is bleed over in your plumbing.
Ensure you have at least the minimum flow rate required to fire unit.
Ensure turbine spins freely.
Measure the resistance of the water flow control sensor.
Check for DC shorts at components.
In short, the more likely cause for a unit that's worked before is a lack of flow; either the device isn't getting enough from a clogged filter, or it's not detecting the flow from a failed turbine.
If you can't get anything to work on the controller (set temperature, etc), then it's also possible the power connection has failed or that the controller itself has failed and needs to be replaced.