Did you make a yeast starter? I'm not sure what happens to the cell walls when they freeze? My guess is that if they crystallize it probably destroys the cell and you would have dead yeast? Having said that, a chemist I certainly am not and thus, am probably talking out of my arse.
Living in a fairly small town in North Dakota I often have to get things mailed to me and last winter I had some liquid yeast sit out on the doorstep a little too long. Some of it did look a little slushy but not all of it. I made a starter and got activity. In fact the beer turned out really good so I'm thinking, so long as you have active yeast the cold probably isn't the problem.
Can you give some insight regarding the recipe, fermentation temp, pitching quantity, wort volume etc. That might help in diagnosing the problem.