Once you hit the lock out point, there aren't any waypoints for act one when you open the map... I didn't get all the way through acts one and two by literally walking everywhere and ignoring fast travel, lol.

No waypoints was why I had to walk back to the elevator to even try going back. I don't want to drop spoilers, but I have a hunch what the "point of no return" is and depending on your methodology playing the game you might trigger it relatively early in act 2 or it could be very nearly the last thing you do... I'm assuming you happen to have done the later and didn't even notice when you got locked out of act 1.