Siemens SE25M575GB/36 dishwasher not starting cycle?

I recently bought a second hand Siemens SE25M575GB/36 dishwasher but it's suddenly stopped working. If I turn it on, I can select the cycle and it appears to start as it makes a whirring noise but it doesn't fill up with water. Sometimes the timer ticks down and others, it just stays at the start time. My husband is a kitchen fitter so has checked all the pipework and water connection but has found no blockages or problems. It wasn't draining properly every time before this happened but apart from that, it just stopped working all of a sudden - an ideas?