You would do well to place a service call or buy a new microwave.
To ask how you might fix it suggests a certain ignorance of how it works. There are dangers of radiation and electric shock inside microwave ovens so take care.
Good Luck...
November 2016
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix microwaves.