I have been periodically burning new files to a set of DVD+R's. I formatted them all for the live file system in Vista which allows such additions. Just now I burned a new file to one of them but for some reason the mastered formatting must have been switched on. The result is the disc now appears blank. No files are visible. Under properties, though, it is mostly full. Is there any way to recover those files? They are obviously still taking up space on the disc. Thanks in advance.

This might be straight, but no.you can't.Well, not from the DVD that is.
I guess you deleted them after you writed them(wanted to write them).
You could install a recovery program like Handy Recovery or Recuva(I recommend Recuva), and if this happened little time ago you can hope that they're still there.Usually, if the files were deleted early, you can recover most of them, but if they're from a long time ago deleted, there usually will be only pieces of it..
Sorry for my bad english
Good luck!:)