When the "Ultimate Drako" attacks the lair, he sends each of the turtles to another world or an alternate universe (Splinter is sent to the Battle Nexus, and Casey is just left in the lair, wondering where everyone has gone).
We see where Donatello was sent in Same As It Never Was. He is sent to a dystopian version of his universe, 30 years in the future. In this universe, the Shredder rules the entire world with an iron fist (literally), and the Ninja Turtles have gone their separate ways. He goes to the lair, and finds it abandoned, and later, after a helicopter nearly finds him, he meets the dystopian Michelangelo.
In this universe Michelangelo had lost an arm at some point (however, he is still incredibly skilled with his nunchucks, possibly even better than his normal self) and is covered in scars. This Michelangelo seemed to have matured a great deal, since he didn't have his trademark humor. He was in fact, very serious.
His voice also had a dry, raspy quality to it - emphasizing the fact that he is indeed 30 years older (which would have put him at about 45 years old). He was very similar to the regular, non-dystopian Raphael. Michelangelo tells Donatello everything he comes to know about this world, such as the disbanding of the TMNT, the fact that Donatello disappeared for 30 years, and Splinter's death.
Dystopian Michelangelo was first of the dystopian turtles to die, killed when the Karai Bots ganged up on him.
Michelangelo's sacrifice was not in vain as Donatello would avenge the deaths of his brothers, and free the world from Shredder.
In Touch and Go, we are told by Splinter that Michelangelo has the most raw talent of the four, but because of his incredibly limited attention span, it's very unlikely he will ever meet his true potential. But, in this episode, the dystopian version of Michelangelo has to devote his whole life to surviving without the Shredder's detection, or he will die - so it is very possible that the Michelangelo we see in this episode is the Michelangelo who has reached his full potential (or, as close as we will get, as this Michelangelo was also very emotionally scarred, and physically impaired (missing an arm).