As the weather is starting to turn cold in the UK, it's a good time to give you some tips about frozen paint.
Firstly try to avoid allowing your water-based paints to be subjected to freezing temperatures. If you store water-based paints in the garage, shed or out-house, try to store off the floor and high up where the temperatures are likely to be a bit higher. It's best to keep stored water-based paints as close to room temperature as possible. Solvent-based paints are much more resilient, although it's worth avoiding cold damp areas that can easily rust metal containers.
Water-based paints left out in the cold don't always have to be thrown away. Freezing does ruin water-based paint, but it can sometimes survive a few freeze/thaw cycles, so you might be lucky!
Before you simply give up on it, here's what to do: Allow the paint to slowly warm up to room temperature (this can be the key to possibly saving it), then stir it well. If the paint colour and consistency appear normal and homogeneous (all the same), then it should be all right to use. If it still looks like cottage cheese, then it was frozen too long and it should be disposed of in accordance with local authority guidelines.