motor keeps running

[BIGPAPPA55]

I,m a newbie, Two week owner of an Ebay special. I don't want to get into that topic right now, I'll vent at a future date. But any how, I just went to turn off my car '83 300D and it just kept running. I remembered reading something about shutting down manually by pressing a stop button on the engine. It worked, but I hope it dosen't continue to happen. What is the reason for this to happen, and what can I do to prevent future occurances? P.S. this is a fantastic site, and extremly helpful to the novice.