Christopher, more details would help to diagnose the 300SE problem. From your description the engine went from starting and running fine to no-start overnight. Surely there must have been some warning signs/noises that all was not right with the engine. If you have access to a spare try replacing the overload protection relay ... located behind the battery ... best to remove the battery and the black plastic cover to access the overload protection relay. Loosen the 8mm bolt that secures the relay, slide forward, then unplug the relay ... install spare in reverse order.

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Fred Hoelzle