try searching on "lost 5th gear" seems I remember something about a dog shearing off..........
This has happened. Not enough for everyone to worry about, but enough you need to take note. Yamaha knows about it, but they aren't telling anyone but dealers. You need to get to a dealer who has done a fair amount of recalls, keeps tech notices from Yamaha, and really knows the bike.
Problem is the gear teeth (if this is what happened) fall to the bottom of the tranny oil pan. They are heavy enough to stay there (but imagine if they don't).
Please check this out really well. Make sure the dealer is checking for this problem.