Frozen or Freeze status is a very annoying thing indeed. This status can be caused by the use of nearly every ice type attack. Pokémon with the ability Magma Armor, as well as Ice-type Pokémon are completely immune to being frozen (with the exception of Tri Attack and Secret Power can still freeze ice-type Pokémon). It can last for a very long time and each turn in battle you have a 10 percent chance of thawing out.
Ice heal medicines are additionally helpful.