At best, there wasn't enough pressure to actually damage anything, at worst you could have a damaged radiator, head, block, header tank, maybe hoses and water pump. in the moderate damage situation, you may just have popped some core plugs. For a graphic illustration of what can happen, put a bottte of water in the freezer...
If you've tried turning it over, while the coolant os frozen in the water pump, it's possible to damage the pump or worse, strip or cause the timing belt to jump, possible causing valve damage.
If you can, get it somewhere warm enough to turn the coolant to a liguid (melt it) and refill with the correctmixture, then check for coolant loss.
kudos for putting your hand up for the mistake - I very much doubt you'll be the only one - even after the lessons you guys had last year!