You should immediately switch off the water supply or gas supply or the power supply to your heater and replace them by installing new ones.
Without further using it immediately hire professional services and replace them.
Stimulate the flushing by pouring a bucket full of water in the toilet bowl as a temporary solution. For a permanent solution you should call for the professional services who will fix the adjustment mechanism but still if the flush does not function you should go for its replacement.
You should clean out the drain completely or call for professional services for replacing it entirely.