1996 E320 WONT START

[jodiedog]

Hello Everyone,I'm sort of new here and here's my problem.We were driving along and we heard a single beep and the lights on the instrument cluster just went haywire. The engine kept running,so we came right home.Shut the car off,tried to restart,turned the key on and the instrument lights came on as usual,but when i turned the key to start,the lights got dim and no start.Checked the battery,it was ok.put jumper battery on just to check it and same thing happened when i turned the key.Iam very familar with MB
I've had 4 in the past and worked on all of them.Im very profifient,but this is the first time I've been stumped. Any help you can give me will be greatly
appreciated.THANKS,Jodiedog.