Your instant geyser is clogged due to limescale deposits, which come from water with high mineral content. During summer time, you should never turn off the water supply to the geyser, or limescale deposits will harden over time.
You can use vinegar to dissolve the limescale. Remove the cold water inlet pipe. shut of the hot water valve and fill geyser with equal solution of vinegar and water let it sit overnight. Next day, drain and keep repeating the process until no more deposits come out. Your geyser will be as good as new.