Summarize:

Set your oven to 300F (150C). The heat will restore most stale bread, although the effect will only last a few hours. Even stale bread still contains plenty of water. It feels dry because the starch molecules have linked up and trapped the water. This means you don't need to add water to the crumb of the bread. If the bread has a hard outer crust, flick water onto it, or run it under the cold tap if extremely stale. If the bread turned dry from overheating, or if it was exposed to air, it has lost some of its water. Wet the whole loaf to restore moisture. This prevents steam from escaping, keeping the moisture trapped in the bread instead. If you dampened the bread, wait until it no longer feels soggy. Depending on the size of the loaf and whether or not you added water, this could take 515 minutes. If the bread is soft but the once-hard crust is squishy, take off the foil. Keep heating for another five minutes, or until you've restored the crust to the right texture. Heat "melts" the starch molecule structure to release the trapped water, but also speeds up staling once the bread cools down. The bread will only last a few hours at most before it turns stale again.
Preheat your oven to a low temperature. Add water only if the crust is hard. Wrap the bread in foil. Heat until soft. Unwrap and heat for another five minutes if the crust is too soft. Eat soon.