Hey mate, first make sure that rain water has not made its way into the engine, either via the exhaust or the intake. Now pull the injectors out, and blow out the cylinders with compressed air. Fill the cylinders with oil, or if you have deep pockets, liquid WD-40, and let it sit for a few days. The next step is to change the oil in the sump and fit a tee piece to the oil pressure sender fitting. Using a small thumb pump, prime the oil system by pumping oil into the fitting until you have pressure on the gauge. The most important part here is to ensure everything is spotlessly clean.
Put the oil pressure sender back the way it was and attempt to turn the engine. You can do this by either putting a socket and breaker bar on the crank pulley and use one arm only to try and turn the engine. An easier way is to "flick" the key switch to start several times and try to shock the engine free. If it doesn't free up easily, you are better off to pull the head off and have a look, rather than damage something made of unobtainium.
Once you get it turning over, wind it for a good 10 or 20 seconds to blow the excess oil out. Now put the injectors back in but before hooking the fuel pipes up, wind the engine over to get fuel coming out of the pipes and pull out the stopper. Make sure the fuel stops flowing because if it doesn't, the engine may run away on start up. If that's all good, hook the pipes up and start the engine. Let it idle at about 1000rpm for a few minutes to get oil everywhere and that's it....
Don't forget to check the coolant and any hydraulics, gearboxes, etc that run off the engine.
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