gas does not ignite

by DAN
(Chandigarh, Punjab, India)

Hello,
my tank less gas water heater was working fine, this one works on the water flow principle, if the water flow and pressure is sufficient, the sensor, powered by two AAA batteries, provides electric spark to ignite the burner, which in turn starts to heat the water flowing through a heating tube, a small LED light displays the water temperature in double digits, two adjustment rotating valves for the gas flow rate and the water flow rate are used to steady it to a desired temperature, however if the cold water taps are opened simultaneously, the heater shuts down or if the temperature exceeds 79-80 C or 175-176 F, it shuts the burner down automatically and will not switch on till the heater has cooled to 20 C or 68 F.
Today after working fine in the morning, the burner has stopped igniting, the batteries, gas flow and water flow are all fine, the settings of valves have been unchanged, when the hot water taps are turned, the LED display lights up, cold water flows out of the tap, the electric spark igniter can be heard ticking inside but the burner doesn't turn on, to repeat myself, the burner is getting both gas and electric spark but it does not turn on, hence no hot water.
Any Suggestions, Solutions are welcome.