In Borderlands 2 on PS3, I tried starting a new game to see what it was like playing as another class, but didn't like having to start from scratch. So, I tried to go back to my original levelled up character - and discovered I was stuck in the new game. I recovered my original character and made it active using multiplayer. But does anyone know a way to recover where I was in the story?
On the main menu, you will have a Select Character option. That lets you switch the active single player save. Once chosen, select Continue.
If you join another host's game, you can also choose which of these saves (characters) to play as in the first screen it shows you after you connect to the server, via the Select Character menu option.
Any progress in single or multiple is saved automatically per save. If you played co-op on the first save and progressed through the missions with others in co-op on their save (mission progression), any you complete with them will be saved on your character and you can't "go back" without creating a new character.
See How does mission progress work in multiplayer? for a description of how MP progress works when playing on other's hosts.