Liens are standard in the construction biz, but they are obviously saying they didn't get paid. Where's your paperwork from FEMA .. hopefully the document that indicates they're putting out the contract for the roof and any other pertinent paperwork? If you got FEMA assistance, I'm sure you got a control or some such #. Contact them immediately on Monday, when you have all your paperwork at hand.
If this company had sent these out with smaller dollar amounts, how many people do you think would pay it rather than risk losing their house?
The Southern Baptist Convention put on hundreds if not thousands of "Blue roofs" during the hurricane relief effort. I helped with about 20 in Punta Gorda. They were free. Now they weren't designed for more than three months or so to get you by mind you.