If heater is on but water is cold, most common cause is burnt heater element (electric) or pilot light going out (gas). Quick technician diagnosis.
See if any of these signs match your device — each one increases the likelihood of this fault.
These are the causes we encounter while repairing Water heater not heating — ranked from most to least common.
Burnt heater element (electric — most common)
Pilot light going out (gas)
Failed thermostat
Limescale buildup on heater
Low voltage
NTC sensor reading wrong
Any of the following = a danger signal — call now and don't try to fix it yourself.
On-site diagnosis, clear pricing before any work, and a documented 6-month warranty.