If the camera back was opened with the film still out of the cassette, the roll has been exposed to light and the images are effectively lost. There usually isn’t any realistic chance of saving the frames once that happens.

To get the film back into the cassette now, remove the roll from the camera and manually turn the spool on the cassette to wind the film back in. You can leave a short leader sticking out if you want to keep the roll for practice loading and rewinding.

What likely happened is that the film didn’t actually rewind into the cartridge even though the tension changed and the counter returned to “S.” The frame counter isn’t proof that the film is fully back in the cassette, so it’s worth checking carefully next time while rewinding for the feel of the film fully disengaging.

Since this roll is spoiled, it’s useful as a practice roll for loading, advancing, and rewinding the AE-1 Program.