R.I.P Peter

Hi everybody when i first came on this website as you can tell my my name i had just gotten a bunny we named it Peter and had him for about a year but then one day when we went to feed him we noticed somthing was wrong he was learning on the side of his cage and when we opened the door he didnt run out like usual so we grabbed him out of the cage to see what was wrong but everything looked normal but if you held him in your arms his head would just fall back and if you set him on the ground he would fall over.My sister is only 10 years old and she took that rabbit in her room and set it on her bed covered it up and layed there petting and talking to it until it died (which was extremely brave of her i couldnt even look at him) anyway we tried to give him food but he wouldnt eat but i know he was trying because right before he died he started shaking and crying. Well that was the day after christmas and since then our family got us a new rabbit she is only 3 months old but i still want to know what happened to peter because just that morning he was jumping around his cage like normal. i cried myself to sleep that night knowing that he died and i couldnt do anything to help him. Do any of you guys have an idea of what it could have been?