Sorry, but I've got no good news for you.

Most pool owners (and many pool service guys) do not fully "blow out" their pool lines when they winterize: it's hard to do with a shop vac or non-professional equipment, like this:

This hasn't be a problem in recent years, because we've had mild winters. However, this past winter was more like what winters I remember from 30 years ago, and many, many freeze damage problems are being reported as people attempt to open their pool.

So what's wrong in your case? I can only guess. Usually, a frozen line will crack and leak and not be totally blocked. One, rather bizarre problem I've seen once or twice: someone jammed a too-small plug into the line, and failed to retrieve it.