Sorry to hear man, thats terrible. It definetly sounds like the ammonia was just too much after you removed your gravel. Your gravel is home to lots of bacteria, and I guess after losing all that bacteria your tank couldnt process the amount of ammonia present. If you try and do that again, try to remove the gravel slowly over a couple weeks, maybe add an established filter to the tank in the meantime. Really sorry to hear though man, its a common mistake that anyone could have made.