No clue without more info....was the tank fed while you were away? If so, and OVERfed, you could have ammonia/nitrite spike.
No matter what it is, I would definately to a fairly large waterchange, 30% or so to dilute whatever is bugging them, and to improve water quality. Lots of fish "heal" themselves when the water quality is upped.
Sorry to not be of more help, but that's not very much info to go on.
Good luck, and post back?