Thanks all, turned out to be a simple fix- the hose had just pulled off the barb on the drivers side of the radiator. I reseated it and move the spring clip to where the hose wasn't indented from it.
Thanks also for the tip on fill-run-fill-run. It took quite a few times of letting the fluid work through before the dipstick continued to read full.
I think that when I backed up in the deep snow, the plastic under guard scooped up a bunch of snow and ice and pushed the hose sideways until it popped off. A good thing to keep in mind for the future- it would have been worth it to dig the snow out.