190D Problems

Hi I own a 1985 190D 2.0L Mercedes it is a great shape runs like its brand new and only has minor dents and scratches. Recently I have been having a small problem with the motor not shutting off after the key turned to the off position. if anybody knows why might be causing this problem it would be greatly appreciated for you advice. One last thing when I had bought this car it had not been driven for about a year i was thinking it might have something to do with the lack of driving?