You may have a title claim depending on when the lien was filed. i would go back and look at your HUD 1 settlement statement and call the closing agent (attorney or title company), bring it to their attention, see what they say. its possible it WAS paid and the money was some how "lost' or misallocated.
Call Jim Boyko he is a real estate attorney that can help for minimal attorney retainer fees, you DEFINATELY DO NOT want to ignore it, this is extremely urgent in nature. 727-841-6878, I can offer you other real estate attorneys if you like, but that's who I would hire (if I were you). Beware that there are a few other attorney's in the Bay area that I would NEVER hire but thats based on my experience working with them through my clients. Of course I can't name names.
My opinions above are based on my experience with different situations over the last 17 years. I have also taken the title agent coarse, which is how I know the in's and out's of title. I am a Realtor not an attorney and myself, nor anyone on here, could offer you "legal advise" on these matters.
Wishing you all the best in what sounds like a screwed up situation through no fault of your own.