We reached the first safe room. During the next area we quit the game. Coming back, there is no option to access the saved campaign. Where is it?
Think of it this way.
Even if you're playing offline, the setup is that of an online game.
Basically what that means, is that however far you get, you get. If you quit, or leave the game, you cannot go back to a "save game". There is no save game. And that's what I love about Left 4 Dead.
In both the L4D games, there's not a "campaign save" per se. Every time you enter a safe room, that's considered the end of one game, and another starts when you leave it to move on. If you quit without having reached the next safehouse, you'll have to start over from the last safehouse.
If you've quit the game, you can resume from any safehouse, though: