1990 300SL m103 refuse to start

Hi all,

Last week my 300SL engine sound like it is running on 3 cylinders so I send it to the workshop. They checked the sparkplug and discovered that the no.6 sparkplug is dead and exchange it. When I wanted to pick the car, the engine started ok then developed the same sympton and then it went dead. The enfine refuse to start and it sound like there is no compression.
Do I need an overhaul? Any advise or experience is appreciated.

Regards,