1) The two resources which are used up as a candle burns are wick and oxygen as they both are reagents of a chemical reaction. It is actually leads us to the reason why the candle runs out every time it burns to the bottom of the wick.You can make sort of small experiment to understand why the oxygen is important for reaction : put the candle upside down in a jar, the flame will immediately run out.
2) Actually, nothing special happens to an atom, but there is a possibility of changing of its electrical charge or causing nuclear explosions. When the energy releases in the form of heat/light/fire, the carbon atoms break their bonds and bond with the oxygen, creating carbon dioxide. That is an important thing which occurs during reaction.
3) It is possible to recover these resources after they have been used as they are not matter or energy which can only change forms but not be composed or root out. Definitely, resources can be recovered but this process usually demands particular amount of time and energy. You have to put back released energy into substance so that it will break their bonds and get them back into regular form.
