My rabbit rabbit died recently; Why?

My rabbit; Thumper died recently. What i first noticed was that he wasn't walking around much. When I let him out for his daily play; he didn't move what so ever, he was just lying on his side. I went over to investigate, when I examined him, his head slumped and he was breathing erratically. upon checking his rear, it was swollen, bleeding and it was completely clogged.