Electric Power comes back when I open up warm tap water and then goes when I close the tap
Sorry I was on a rush. I'm in Germany and the device is called "Vaillant Durchlauferhitzer electronic VED E 27/7 Elektro-Durchlauferhitzer 27 kW" heizungsdiscount24.de/durchlauferhitzer/… 


Your heater is a three-phase load, most likely with the heating elements connected between the phases.

I'm not an expert on german wiring practices, but my guess would be that your heater has developed a short circuit in one of the elements, said fault blew out the fuse supplying one of the phases and is now back-feeding said phase. Whether said fuse is your responsibility or your electricity suppliers responsibility I could not say.

I would advise against operating the heater until it has been inspected by a competent service technician and the supply fault has been rectified.