Steering tube turning inside fork crown
There are some MTB forks that have replaceable steerers that are attached via binder bolts. Here's a Marzocchi and an RST one, respectively:

If your fork is like this you may be able to get it fixed by just tightening the bolts. You should probably take them out and re-loctite them first if so. However, look through the crown area carefully for any other damage that might have occurred from the crash, such as cracking or deformation, either of which could produce the steerer spinning as a symptom (and it would seem kind of likely there's something like this going on, given the timing).
If your fork doesn't have a steerer attachment like this, as most don't, it's more or less a goner. If it was bonded or retaining-compounded in to begin with, you maybe could fix it by re-bonding it, but I'm not saying this is necessarily a good idea.