Certified GKH Gardening Expert
That is so disappointing. It happens often when trying to rebloom an amaryllis but the first time around is especially disappointing. Always buy a high-quality large bulb. An inferior bulb may not bloom. Also, if the bulb did not receive its ample rest period, it won't bloom.
If the bulb is watered too much, or there is not good drainage, it can rot. The bulb should sit above the soil by 1/2 to 1/3 of its height. It should sit in a sunny spot and be watered when soil begins to dry.
If all of those conditions were followed, I would chalk it up to the bulb. Since the leaves grew, it doesn't sound like the bulb rotted so you can try again next year.