Important distinction: heater not heating (water lukewarm) vs no hot water flowing at all (no water). If latter, the hot water line is the issue, not the heater.
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 No hot water at all — ranked from most to least common.
Inlet valve closed
Blockage in water inlet to heater
Mixer valve in bathroom faulty
Cracked outlet hose with air
Instant heater: pressure below activation
Clogged internal filter
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.