Have you fallen in Arena quite a bit? Removed some Faction Guardians?

no... in fact I have just recently gained gold status in the arena (classic) and all my faction guardians are in place.

Accidentally removed gear from someone important?

Gear removal was free recently so you may have inadvertantly removed it. You could have also accidentally reset their masteries. I would check those two first. 

Yeah, I think we all occasionally move a piece of gear and then a team doesn't work. Then you just have to dig through everything to see what you changed, I know I've looked over a missing piece of gear a few times before myself. :D

So go back and quadruple check and if that doesn't confirm anything, take screenshots of every champ in the teams and their gear and masteries and we'll see if we can help more.