Either the high voltage (4000V) fuse has gone which indicates that there is a problem with the magnetron, or depending on the design of the circuitry the door safety intelock switches have gone either way a job for a tech.
Whatever happens please remember you are messing around in a piece of equipment that runs at 4000volts and due to the design of its circuitry under certain fault conditions it remains lethal months after having been disconnected from the mains supply. its not rocket science as they say, but it is not for the layman either. Contact me via microwave-repairs.co.uk if you need further help.
patrick
September 2009