Are you having any type of connection issues? Can you play other games online? Once the error pops up for you, Bungie is notified if I'm not mistaken and they will try to solve the issue once it's seen. Other than that I'm not quite sure what else you can do. If you are on Xbox you can quit the game on your dashboard and try again. And if that doesn't work maybe hard reset by holding down the power button until it goes off. If neither of these things seem to work or if you are on PS the. I would suggest simply being patient and waiting it out and a solution will come eventually.