I think the 928D is a diesel. If possible, try to turn the engine over by hand, very slowly. Don't force it. If it can't be turned over it may be hydro locked with rain water from being outside. Remove the injectors and again turn it over by hand. If it won't revolve by hand with the injectors removed, the piston rings may be rusted to the cylinder walls. The engine will have to be torn down to repair.
If it will rotate by hand, try to spin it with the starter. If it spins easily with the injectors out, change the oil and reinstall the injectors, bleed them, start the engine run it for 15 minutes or until it reaches operating temperature and change the oil again.
Don't leave it outside.
edit: I've never tried this but wonder if it would be possible to adjust the exhaust valves so tight that they are held off their seats by .050" or so and SLOWLY rotate the engine by hand to allow any liquid that may be in the cylinders to escape through the exhaust valves and into the exhaust system. If it works, that would make removing the injectors unnecessary. Once the liquid is gone, adjust the exhaust valves and start the engine.