I played b4. According to computer, it is a "missed win" move by me. But what move did I miss? What move should I have played? (I am white. And played b4 but I got "missed win")
After 23...Ra6? White has the option of going for an immediate kill, as black's king is in danger of getting checkmated: 24.Re7+! is a very strong move, forcing black to play 24...Kc8 (since the reply 24...Kc6?? would be immediate checkmate after 25.c4! followed by 26.Rc7# whatever black plays).
After this, white may play 25.Rc7, after which black is practically forced to play 25...Kd8 (if black goes 25...Kb8, white can play 26.a5! after which the simple plans of Rd7-d8# or Ne7-Rc8# are quite crushing) and in this situation, white may choose a few different winning plans. The most basic one is to just take pawns by either Rxb7+ or Rxf7+ using the fact that there is a discovered check. But the most crushing move seems to be 26.a5, after which black will lose at least the exchange, if not more.
A sample line: 26...Ke8 (stepping out of the pin) 27.Rxb7 (threatening Nc7, and the rook on a6 is trapped; there is no way of preventing the move, and black loses the rook on a6 for at most the bishop on b6.)