how do i fix my shower pump?

hi there, the shower pump we have as stop working, its a WATERMILL PK50D. it started to loose pressure yesterday, and now it will not come on, and i am getting no hot water coming from shower or bath tap .i am getting cold water through cold tap at the bath. and all the other taps seem to be fine. the is power getting to the pump. because it starts up, it just wont kick in makes a humming sound, i also cleaned out the filters on the inlet connection. But still no joy. some one said that there might be air in system that is why your getting no water, i did the hose on the mains tap and through the bath tap, but it just went in the tank and through the overflow pipe, any help would be great, many thanks for now,