One night I drove my vehicle 2 miles to the gas.

One night I drove my vehicle 2 miles to the gas station with no problems, pumped some gas and then tried to start it back up. When starting the vehicle it makes one single clunk, like the starter is bad or perhaps the flywheel isn't able to rotate. I have tried jumping the car to rule out the battery, I have replaced the starter, and I have tried to turn over the engine using the starter with the spark plugs removed to rule out hydro-locking. These changes have no effect. At this point I figured my engine is seized (although it has not lost any oil and the dipstick shows full) since I can't turn over the engine using the crank shaft pulley in the clockwise direction. But interestingly, I am able to turn it in the counter-clockwise direction. While doing so I can see the pistons moving in the engine (through the spark plug hole). I have turned it 13 complete revolutions. Any ideas on what happened?