Turn off electricity and try a multi meter over the pump to check the resistance. Then so long as you know what your doing and have the right equipment test the pump voltage live.
But I emphasis you must know what your doing and how to test safely observing all safety precautions. If in any doubt at all call a qualified service engineer.
There is always the possibility a component has failed and its not calling the pump, but that would show up when you test the pump voltage. If there is no voltage when the system is calling for the pump, then obviously the problem is further back along the command chain.
What we suspect is that the banging is actually water over heating in the DHW heat exchanger and the pump isn't working to circulate and cool it.
Hope this helps.