Pour a few tablespoons of Automatic Transmission Fluid (ATF) down the spark plug hole into the cylinder. Leave it sit with the ATF in it for a few days. With the spark plug still removed, crank the engine over a few time to expel the ATF from the cylinder. Turn the key off, spray a light squirt of starting fluid or carb cleaner down the the spark plug hole into the cylinder and install spark plug. Try and start as you normally would.

Soaking the cylinder a few days with the ATF should dissolve any rust/corrosion that may have frozen the piston rings to the cylinder wall. If still will not crank over, then I would suspect the starter is corroded or bad. Of course it could also simply be a bad starter relay. To test this I would use a jumper wire from the battery positive side to to the starter side of the starter's relay. If the starter whirls to life, then this would confirm the relay needs replaced. If the starter still will not engage then you're back to either a bad starter or the piston being froze in the cylinder.