1. RELAX. Everything's going to be just fine. This has happened once to just about everyone. And here's a good place to interject my favorite phantom statistic, 9.9 out of 10 times the beer turns out just fine.
2. Fix another sanitized bung and airlock to the carboy and allow the primary fermentation to be completed.
3. Proceed to siphon it into a secondary like you normally would (if you are single stage fermenting then wait until fermentation is fully completed and then siphon to a bottling bucket and complete the bottling process.
4. Once the beer is in secondary or bottled you can dump and rinse out the trub that has collected at the bottom of the carboy with the bung in it.
5. Next invert the carboy and guide the trapped bung into the neck of the carboy so that it is as close to the opening as possible.
6. Then take a smaller screwdriver and insert it into the carboy opening and through the hole in the bung.
7. At this point it will serve you well to breathe deeply, remain calm, and BE PATIENT.
8. Once the screwdriver is through or at least up into the hole in the bung you can, in fulcrum fashion, CAREFULLY push the handle of the driver up against the lip of the carboy and force the carboy bung back out through the opening. **Note: Please understand that this takes a little doing and you will need to be persistent and careful while doing it, but it does work.
9. After extricating the bung you can be fairly certain, by the very unpleasant nature of the whole experience, that you will not do that again!
10. Be aware that you do not have to force the bung into the carboy as hard as you can. Just push it in so it has a good seal, and then leave it alone. If you are having a problem where the stopper keeps trying to pop out, it is likely that the stopper is wet. Try to dry the stopper with a clean towel and insert it back into the carboy. The rubber stopper forms a tight seal without a lot of pressure being forced into it.
11. This is also a good time to interject how important good sanitation should be in place. Anything that even has the possibility of coming in contact with your beer or wine needs to be sanitized thoroughly, so make sure you take the time to get everything in tip top shape. Otherwise, you might be in trouble at this point.

You can also try the towel in the carboy trick. After removing all of the liquid, slide a towel into the upright carboy, turn it upside down and allow the stopper to fall near the neck of the carboy. Slide both items out at the same time.

Best of luck!