Well, something went wrong there. I'd definitely start with a factory reset, but if that doesn't work, I'd say you're looking at flashing back to 4.4.4, then working your way up to 5.1 instead of skipping. That could be the problem, but I'm still hoping the reset will iron out the kinks that this has caused. You should be able to skip, normally, but since this was a major refresh, I suppose it's possible that that's the problem.