1999 Ford Taurus Master brake cylinder

So I noticed while parking the other day that my brakes went to the floor. I parked it and pumped the brakes a few times and let it sit to see if it was leaking anywhere on the ground, it was not, not that I could see. I checked the resavore and it was still full. I check on the inside of the rims and nothing. I can sit and pump the brakes and they dont stiffen up like they should. I was wondering if it could be the master cylinder or something else, air bubble, or something else? I also did not find a leak under the master what could this be? It is cold here wondering if something froze?