I just got back into Medieval and this was my second attempt with the Saxons.. The first attempt failed as the Mercians got too strong and overwhelmed me, and my provinces kept revolting with rebels.. (on hard setting).
I learned from this on my second go and kept upgrading my regions to stockades and recruiting some peasants, etc. My empire was expanding, expanding gloriously (richest kingdom).. I killed off the Mercians, and had almost killed off the Welsh. The Northumbrians didn't look so strong. I thought I was approaching end game.
Then, I suddenly got a message saying that my King had died from illness and that I had no heirs. Boom! End of game!