Touchback is correct. The rulebook has a defintion called "Impetus". It's who is responsible for putting the ball in the end zone. Impetus is charged to the player who passes, runs, fumbles, bats, or kicks the ball into the end zone. A player that muffs the ball does not fall into those categories (unless the ball was completely still and at rest when it is muffed), so in your play, the impetus is still with the player who kicked the ball.
In the case that I bracketed when it's at rest when it gets muffed. That is the one time a muff would change impetus.