Just few days ago, I have the similar problem. The water heater suddenly stopped working. My water heater is Stiebel Eltron AQ 35E. It's rated 230V at 3.5kW. The electrical cable is 4mm. I opened the heater and measured the power. It has power all the way into the heater. But the heater just won't turn on.

Any ideas?

It's broken.

Flow switch? Over-temp switch? On-off switch? Clogged heater? Heater control electronics (if it has them)?

Could be any one of a dozen things.

If you don't know what you're looking at leave well alone, it is quite capable of killing you.

Call the man or buy a new one.

EDIT As this device has a built-in RCD try pressing the 'Reset' button.

I tried the Reset button, but no response.