How did my bunny die?

Last night my bunny,Ruby was just fine. He was happily playing around with my other bunny, Sunday. I originally got him to be a playmate for Sunday, but i starting loving him. Then this morning I found Ruby laying on the ground dead while Sunday tried to cuddle with him. I felt so sad watching that. Now Sunday looks sad and lonely. All it does is stay still or cuddle with me. I miss Ruby sooo much and so does Sunday. Now im wondering how he died so it doesnt happen to Sunday. He died next to my desk. He could have fallen off the desk(its high) but my dad doesnt think so. Ruby was about 10 weeks old when he died. How did he die?? I feels like its my fault and now i feel so guilty...poor bunny