A blank tankless water heater display can mean the controller is off or power is unavailable; it does not by itself identify a failed control board. Check the exact model's normal display behavior and safely accessible power status. Record any recent outage or service, and leave internal wiring, gas and safety-device tests to qualified technicians.
Compare the display with the owner's instructions
The cited Rinnai RSC SENSEI manual says the display goes blank when its controller is powered off using the On/Off button. The cited Navien NPE operation manual identifies its front-panel Power button separately from its diagnostic and information buttons. Use the instructions for the actual model; button names and normal screen behavior vary.
If the heater was intentionally switched off for maintenance, do not restart it until that work and the manufacturer's return-to-service checks are complete. Ask whether the water and gas connections were restored properly. A clear screen does not authorize turning an unfinished installation on.
Check accessible power status safely
Note whether the house lost power, whether other equipment is affected and whether an identified heater disconnect was left off. Look only at safely accessible controls and connections. Do not reach into a wet cabinet or use an extension cord to supply a gas appliance temporarily.
Repeatedly resetting a breaker that trips is not a useful power check. Keep it off and arrange qualified electrical diagnosis. Likewise, leave voltage measurements, fuses inside the appliance, control wiring and board replacement to trained service personnel. The heater's front cover is not a troubleshooting access point for an owner following this page.
Separate an unlit controller from cold water with a working screen
If normal power and display return but the hot tap stays cold, record the error message or operating indication. Why you have no hot water covers the wider water-heating problem. Common tankless water heater problems separates flow, ignition and other faults.
For a Rinnai code, the Rinnai error-code guide can help identify what the display is reporting. Do not apply a reset intended for a listed code to an appliance with a different electrical or gas fault.
Tell the technician the model, whether the screen ever lights, the last operating mode and any outage or recent service. Those observations help separate a supply interruption, controller setting and internal defect without purchasing a board on suspicion.