The toilet in an upstair bathroom does not have water in the bowl but the tank is full of water. What could be the issue? Do I need to replace the whole set? Thanks.

Our tub normally drains like a champ; when showering, the drain has no problem keeping up with the water entering it. My wife took a shower this morning, and there were no problems. This evening, after pulling the kids out of the tub, I opened the stopper on the drain. About 5 seconds later I heard a very loud pop from the bathroom sink, followed by a strong smell of sulfur. The tub is stopped solid - it's been standing water for over 2 hours now and doesn't look like it's going to drain anytime soon. I checked the toilet, bathroom sink, kitchen sink, basement bathroom, etc - and everything runs fine. Yes, even the sink in the bathroom that origrinally made the pop sound and the terrible stink afterwards drains fine. Given the abrupt nature of the clog in the bathtub, the loud pop, and the subsequent smell - I'm at a loss of what would cause this problem. Any suggestions, ideas, or obvious answers?