Motor / engine keeps running

I just bought a 1983 240D. It ran great until last Friday. I had a little difficulty turning the ignition but finally got engine turned on. Then when I turned it off the motor kept running, even after the key was out. It was difficult to turn off even with the stop mechanism under the hood. I have only had the car a couple weeks. The owner said it ran fine and I actually drove it a week before buying it. It ran great up until Friday. HELP! I am afraid to drive it (I actually won't drive it until fixed.)My husband has checked the hoses and they seem to be fine. Any suggestions on where to go next?Is this a common problem with diesels?