Add sugar to bottle-conditioned beer
Warning: This method is error-prone, high-effort, and probably not worth your time. Could be fun, though. :)
Most commercial beer is force-carbonated. That is to say, the beer is produced and ready to drink (minus the fizz) before they put it in the bottle and mechanically carbonate the beer.
Beer that is bottle-conditioned or bottle-fermented, however, is put into a bottle with some extra sugar. The beer is intentionally left with a little of the yeast from the fermentation process, and is sealed with the sugar. The yeast then does it's normal thing: it eats the sugar and excretes alcohol plus carbon-dioxide. Since the bottle is sealed, the CO2 has nowhere to go, so it goes into dissolution in the beer, giving you that fizz. Most bottled homebrew will be like this.
So, if you have a beer that is both flat and was bottle-conditioned (and therefore still has some yeast in it), you might be able to bottle-ferment it again:
- Add a very small amount of sugar to the bottle. Table sugar works, although corn sugar (which you can find from a brew store) is ideal. It'll be really hard to get the right amount (about 0.083 oz), so I recommend pre-measured tablets of the stuff: http://www.northernbrewer.com/shop/munton-s-carb-tabs.html
- Seal the bottle somehow. If you have a bottle-capper and some caps, that'd be ideal. Otherwise, you might be able to get by with a soda bottle and cap. If you're really lucky, your beer is in a swing top bottle.
- Wait a week, maybe a couple to be sure.
If you're lucky, there's enough viable yeast in the bottle to start acting again (now that it has new sugar to eat), which will create more CO2 and recarbonate the bottle.