There are a number of interlocks on a microwave to prevent it operating when the door is open and to start/stop the fan. It may be the door interlock switch has failed (Seems to be a common problem with some makes)
It could also be a problem with the control board but someone competent should be able to check the interlock switches
http://www.neff.co.uk/store/spare-parts
acp, April 2017
