i stacked a wedding cake last night and the 2nd tier has developed an air bubble and blown a hole in the fondant how do i repair it.

When I have this problem I just poke a very small hole with a pin or toothpick and gently press on the fondant bubble to release the air. :)

Nevermind I saw where you said it developed a hole in the fondant. yikes! Could you unstack the cake and recover the tier? I can't really think of any other answers but I would love to see the answers from more experienced decorators :)

i have never taken fondant off a sponge cake is it possible to do it without damaging the sponge?

I have done this but need to repair the large crack in the fondant

You can repair the crack several ways using either fondant or royal icing.  I prefer using fondant because it will match very well. 

I take some well kneaded, warmed fondant (nuke a small amount--like 2 ounces-- at 50% power for30 seconds--  and then add in a few drops of hot water at a time to it until I can pipe it. 

I pipe some into the crack, smooth it, and let it dry.  It'll shrink a bit, so I usually need to do it 2-3 times, depending on the depth & width of the crack.  If you get excess on the cake surface, use a palette knife to gently scrape it away while it's wet.

Royal dries a lighter color than it's tinted, so if doing something other than white, it can be tougher to match, but it works.  Pipe it into the crack, let it dry, and then see if it needs more.

I have done it in an emergency using tinted buttercream.  You need to be super careful because the oil in the BC will stain the fondant, but it works in a pinch.