Well, I don't know what you would need to do for sure...but I do remember that you are in CA...I wouldn't imagine that the snow will stick around for long, but that too depends upon just where in Ca you are. I guess if it was me, I would see about trying to clear a spot for the horses. Do you have a tractor??? I would clear off part of the arena for now...Maybe not down to the dirt but take the first couple of feet of snow off. Then the horses would be more able to lay down if they wanted to. Rest will be important to them since their barn is gone and the snow is up to their bellies. They can sleep standing up but they also need that deep REM sleep that comes with laying down. Also feeding will be a pain in the rear now, as well as keeping a thawed water supply. Keep more than ample hay available for them right now. They will need the warmth that the extra roughage will provide.
I wish you the very best of luck and hope for a quick melt. I'm really sorry to hear that the barn collapsed. That's just a stinker of a situation. Blankets will help and if it makes you feel any better, my horses don't even use the barn, they stand out in the ugly weather and they are just fine. Horses are a bit tougher than we sometimes give them credit for.
Be strong and courageous, and do the work. Don't be afraid or discouraged by the size of the task, for the Lord God, my God, is with you. He will not fail you or forsake you.
1 Chronicles 28:20