5hp 4 stroke won't crank, won't turn a bit

I bought a used 5 hp 4 stroke Nissan in January, before I bought it the guy started it and it ran well, I brought it home, started it, ran the gas out then took out the plug and sprayed in the storage spray and pulled the starter cord a few times to spread the stuff around then the motor sat in the shed till now. I also changed the engine oil and the lower end oil. I live in Edmonton so it gets below freezing here but when I did all this we had a very warm spell and it didn't freeze for a week. Now the motor won't crank, I pull the starter cord and all that happens is the cord comes out a few inches till it engages then everything stops. I had the motor stored upright in the shed then took it out to the lake laying on its handle side as the manual says. We made sure the engine was in neutral, also pulled off the starter cord assembly and tried to turn the flywheel with no luck. Pulled the sparkplug with no effect, checked that the valves were free and could press them. If we put it in gear and try to turn the prop, the prop turns a bit indicating the problem isn't in the lower end. Any suggestions where to go from here?

Possibly oil leaked into the cylinder-----Remove sparkplug and aim sparkplug hole down and see if oil comes out of sparkplug hole..

Thanks everyone, we had the plug pulled and I changed the gearoil in the lower end about 3 weeks ago - what came out was dark but no sign of water contamination. Guess my next step is to drop the lower end but if you try to turn the shaft when in gear you can feel the driveshaft torque up.

Either the driveshaft is stuck, or the motor is stuck. Once you drop the LU, you will know which.

Ok, dropped the lower foot and the drive shaft stayed in the engine, pulled out of the foot. The engine still does not crank and I don't know how to get the drive shaft out. I suspect this is why the guy sold the engine because the impeller looks new but I can't find the pin that's supposed to secure it to the drive shaft.

any suggestions on either problem?

The bearing is supposed to be pressed on the drive shaft, and that should be captured under the wp housing. Is the motor stuck now? You will need to either see a dealer or disassemble the LU to se what is wrong.

Motor is still stuck. I have removed it from the housing turned it upside down and am soaking the drive shaft to motor joint with penetrating oil. Did slide the foot back on while the motor and housing were together and it slid on easily, no press required.

Should I be pulling the cylinder head first or the oil pan to check the bearing?

You will need to pull the power head, and see whether the drive shaft is just stuck in the splines, or if it's twisted/damaged. You will also need to disassemble the lower wp housing from the LU to see what is what there. The roller bearing below the wp housing is supposed to be pressed on the drive shaft. Once the power head is off, you may be able to assess whether the piston is just stuck, or if the motor is seized from lack of lubrication. If the latter, bearings will be bad, and you have a lump of scrap.

Can I drop the oil pan over the drive shaft to get to the crankshaft/driveshaft joint? That would also allow me to figure out where the seize is and if it is worth spending any more time on.

No need. The crank extends out of the lower seal.

Great, the engine is seized so by dropping the oil pan I can get inside and look for the problem. That will also allow me to apply some direct force to the joint as all I can do right now is tap the shaft or sides of the seal area.

Found the problem! When I took off the crank case head, where the driveshaft meets the crankshaft, a whole bunch of white crud fell out, the area was packed with it. Now the engine turns freely but the drive shaft is stuck in the crank case head. I have got out two seals but there is still something in there, don't know if it was assembled wrong and the spacer is in the way as I have not pulled out a spacer yet. So now I am further along but still don't have a clear path. Any guesses what the white crud is? Could it be an accumulation of oil water mix if the area was getting contaminated? Any guesses at what will get rid of the stuff as I can't seem to get in there.

The white crud is probably corroded aluminum. Sounds like there was water in there. If you clamp the die from a flaring tool onto the drive shaft, you can then probably use wedges to get the shaft out. Of course, you will need to clean the head casting, or maybe replace it, and install 2 new seals and the spacer. Now would be a good time to see how in the world the bearing came off of the drive shaft below the water pump.

I am going to cut off the head casting and just buy a new one.
I tried to get to the bearing in the lower end, and the bolt that holds on the lower part of the water pump snapped off flush with the top of the metal after it came out a couple turns. I tried to seperate the housing from the lower foot to get at what is left of the bolt but the gasket must be glueing the parts together. Any suggestions on how to get the bolt out or the parts separated?
on the bearing side of things, once I get it out, do you know where it is supposed to sit on the shaft? I will get it pressed back on, just need to know where to stop.

See diagram below. The wp upper and lower housings are a sandwich, and the lower fits closely into the LU casting.

Yes, have that apart, it is the lower casting stuck to the rest of the lower end. Can't seem to get it off.

Ok, got the parts out, now I have to press the bearing back on the shaft, where does it sit, dimension from bottom of the shaft to either the bottom or top of the bearing, please just let me know which. Thanks for all the help