I have a 1991 mercedes 300sl on a J plate done 120k full history,

I have a 1991 mercedes 300sl on a J plate done 120k full history, it worked fine for a couple of weeks then one day i tried to switch it off and i turned the key to off but the car kept running ,it did this for about 5 minutes then stopped, while it was still running i was able to remove the ignition key as it was in the off position, since this happened,intermitantly it wouldnt start and no lights would appear on the dash,then after turning the key on and off for many times the dash lights worked and it started and ran fin, then after turning off it did stop but then wouldnt start again until the dash lights came on, it did this each time i tried to start it over a couple of weeks as i wasnt using it but tried to start it regularly but now it wont start at all and i cant get the lights on the dash at all now so i'm kind of stuck and need some advice many thanks phil