It was the interlock switches. I don't recall if it was one or two. They are trivial to replace. It's been awhile, you unscrew the panel that is behind the keypad and you will see the interlock switches. Most likely it is the topmost switch but check them all with a meter. You can either order replacements online or take them to your local electronics parts store (for example, Frys Electronics) and get replacements. I keep a few around because I had to do this again last year. The switches are inexpensive, a few dollars each I think. I have heard that stopping the oven before opening the door instead of popping the door helps keep the switches functioning longer.

Replaced top interlock safety switch (goes in at an angle) and blown fuse, microwave now ready for another 15 years. The interlock switch had a broken spring inside and would not open, causing fuse to blow.