I mixed up a bottle bucket full, put the lid on it. got busy with other stuff, and a few beers, left the bucket out in my unheated shop over night. it dod not freze solid but did have ice on top. do I dump it or use it?
Personally I would guess it would be fine - starsan is basically acid plus surfactant, and I would guess that freezing it wouldn't change anything. If you're that worried you could just mix up another batch - if you buy it by the bigger containers it costs about 10 cents a gallon of mixed solution, so it's not like it will cost you much.
Send the folks at 5 Star an e-mail. I have heard that they are very quick to help if you have a question about one of their products. I would also check the ph like was suggested above. As long as the ph is good, it is supposed to be good to use.