Wow, This just happened to me Saturday. I hadn't painted in some time, being ill and medicine making my hands very shaky, I couldn't do any detail work, so I stopped. All better now, thanks! But, I knew Citadel paints were water-based so I did what you mentioned. Dropped in a few drops of water and stirred the pots up. Only a couple were left lumpy. I was able to smooth most of them out. Of course, since I wasted so much time restoring pots of paints I didn't get to finish my mini, but hey, I saved about $40 on paints. Somone suggested using a nut or bolt in the paints to assist in mixing. If you do this make SURE it won't rust. Rust will ruin your paint. I suggest a marble. Anyway, best of luck to all, and Happy Painting and Gaming!
<edit>Heh, RogueJK beat me to the punch, but hey, now two people have said it so the voice of experience is showing through. Plastic, glass, clean pebble, anything besides metal to mix. 'Nuff said. </edit>