Hi Sue,
There are one or two most likely answers, the first is that on opening or closing the door, or when pressing "start" the thing went pop. Sometimes this can happen during microwave operation.
The second and more costly option is that either the magnetron, or possibly the high voltage capacitor have failed. Both are really jobs for an engineer.
My charges for the first are approx £39 inc and for the second around £75 to £85 for the second. As a guide to any quotes you get. My inspection fee is £10.
Contact me via microwave-repairs.co.uk if you need further help.
Patrick
October 2010