From Blaseball Wiki
Frozen is a modification that temporarily freezes a player, making them unable to bat or pitch until the end of the game.
During a game in Snow weather, any player has a chance to be Frozen. This completely stops them from pitching or batting, like the similar modifications Elsewhere and Shelled. Unlike them, however, Frozen wears off after the end of the game.
Strangely, players Frozen while on base were able to be batted in, and have even stolen bases while Frozen.
Frozen was first seen on Day 1 of Gamma 3, as a result of the Circuit 00 Decree adding Snow weather.