If the flushing mechanism is a "syphon" type then a) it may have become faulty or b) there simply is not enough water in the cistern to usefully operate this system - cistern based syphon flushing systems are almost completely unknown outside the UK and its possessions/colonies just as bowl based flush systems are a USA design and almost completely unknown outside USA and Canada - parts of the Middle East import both. For b) check the water level whenever flushing does not work, if it is high then a) is the most likely cause. Unfortunately, to my knowledge these things are not generally repairable unless you discover that a tube has become partly or completely detached and can be refitted or replaced.