The starter motor when switched engages a pinion gear onto a toothed wheel (ring gear) adjacent to the flywheel (or part of the flywheel); the ring gear can only stop in one of three or four places. That means when the starter motor is switched the pinion gear on the starter motor only ever engages with 3 (or 4) places on the ring gear (the same places over and over again). If at one of these points the ring gear is worn (missing or damaged teeth) the starter motor pinion gear will not have anything to engage with on the ring gear and the starter motor will not turn the engine. It is worth examining the ring gear (from underneath the car and looking from the top down the side of the engine to get the best view possible).
However bearing in mind that the engine has been sat for 2 months it is possible that the engine has seized (possibly some rust build up in the cylinders), that said it should be relatively easy to free it up. With the spark plugs removed spray some penetrating oil into each of the cylinders and leave it to soak for 4 hours. Then try to turn the engine using a spanner on the crankshaft bolt / nut. If it turns give it half a turn and then put in some more pentrating oil after soaking for 4 hours turn another half turn using the spanner; if the engine is free give a further full turn and then try the ignition (after replacing the spark plugs of course). If it doesn't turn then spray in some more penetrating oil and leave to soak overnight; then try with the spanner. If that doesn't do it I think it might be time to visit a garage. If it works I strongly recommend an oil and filter change.
Andy