This, especially the line in bold. If they are solid, they are gone.If they have set hard they are pretty much dead. If they have just thickened up they can be rescued. If you have a couple of pieces of off cut white metal from a sprue lying around, put them in the pot, add a couple of drops of warm water, and shake the shit out of it. Bare in mind only add a couple of drops at a time, so you don't make it too watered down.
My only advice would be to use medium instead of water to mix them back to life - It gives a better consistency and the paint molecules doesn't split up so you get a paint that is as close to a stock paint as you can.