I've got a 2 group and im having the exact same problem (on one group only, the other one wors fine). It only started today, although it happened to me about 12 months ago and some how fixed itself. i just pulled off the top and got a replacement switch from jaycar but unfortunately its not the switch. i'm just hoping its not a solenoid issue.
mine is also making the humming sound of the pump as normal to only thing is the water isn't coming out. ive just turned it off to cool down and once thats done ill check all the connections within the machine.
any one had this problem?