Sylvania Microwave Oven - SC-11100

Could be lots of things. If you have the manual, it may help to look up info about special modes (Sabbath day mode, for instance) and control locks and such; perhaps you activated some feature that you didn't know about.

Otherwise, I'd guess something is awry in the control circuitry or the keypad itself: either a button that's pressing itself due to wear, or a loose connection, or perhaps a capacitor in the digital power supply that's aged and is causing noisy power rails for the controller, or something of that ilk. Chances are it's not particularly economically viable to repair if you don't have the skills to do it yourself. (And do be aware, if you're thinking of troubleshooting, that microwave ovens are among the most dangerous household appliances to work on if you don't know exactly what you're doing. The magentron supply is designed to output a nontrivial current at several thousand volts, and presents a very real electrocution hazard; and portions of it may remain charged even when things are turned off and disconnected, doubly so when something is broken. That's not to say it's impossible for ordinary people to safely work on microwave ovens, but rather to make sure you sufficiently research and understand the dangers and safety procedures involved.)

Here's the manual:
SC-11100 manual

Unplug it. Wait 5 minutes. Plug it back in.

If that doesn't solve it, go to a store for a replacement unit.

Drew reminded me of my experience with a dead microwave in the 70s when they had first come out. I'd bought one from Sears and after about a week, it made some arcing noises and stopped working. It took the Sears repair depot a few weeks to figure out what was wrong (they'd never seen one before) and after I picked it up, a few days later the same thing happened. Back it went. This time they fixed it quick (now they knew what was wrong..it was a failed high voltage rectifier). Got it back and the FIRST use it made the sound again. I yanked the cord from the wall to try to prevent any more damage. A good friend and I had recently finished up electronics courses at different schools and of course we each felt we had learned more. The tamper seals were long gone by the Sears shop fiddling so Marvin and I decided to look inside to see if we could see what was wrong. We were stumped as everything looked fine. Then Marvin said let's plug it in and turn it on and see what's arcing! I reminded him that without the covers on, we'd likely be cooked by the microwaves. His response... "We'll wrap ourselves in aluminum foil and watch it through a mirror!". My cooler head prevailed and after removing the high voltage protective cage, we could see where one side of it was "oil canned" in and there was a black spot where the high voltage was jumping from the rectifier to the cage. Simply pushing the cage out so it was "oil canned" away from the terminal rather than toward it cured the problem and I used that microwave for at least 15 years after that!

The first thing to try is a "hard reset". Unplug it from the power line for a few minutes, plug it back in, set the clock, then try again.

If it still doesn't work, it may be a bad door switch or door switch mount.

One quick check is to gently hold down on the door while you press Start (or Cook).