This sounds like a fault with the element, either the element is shorted resulting in to much current being palled through, or leaking to earth, this should be a relatively straight forward fix, the part is around £15.
Microwaves are high energy appliances so it is advisable to get this repaired professionally.
If you would like me to recommend an engineer please advise of your post code, please get in touch if you require any furthre assistance, however to better answer you please provide the model number of the microwave.
We would normally recommend someone on the approved Panasonic service centre list, however I know of someone in the area who is near by and is able to come out and take a look, the usual charge is £30 plus parts.
Pete's phone number is *****
An alternative engineer should Pete not be available is Les' Domestic Repairs, and his number is 07900 593540.
Please let me know if I can be of any further assistance.