To try and revive, some medium, thinner and water will help break it up. Add a stainless steel ball bearing and use an electric shaker for nail polish to speed up the mixing process. If that doesn't bring it back then just buy new ones. For medium just check Amazon or Hobbycraft for artists acrylic medium (gloss or matte) and get it much cheaper. I get 500ml bottles for around £8. I also use flow improver for airbrush and brush painting. Flow improver - helps paint flow without discolouration or changing transparency. Thinner - Will thin paint and make it more transparent. May cause some colourshift. Medium - Will maintain thinkness/flow but will make paint more transparent. Paints are basically coloured particles suspended in clear medium. Lahmium and other mediums are just the backing product without the particles. For working longer projects I would also suggest some retarder which acts as a medium but slows drying time allowing you to wet blend longer or work with the paint longer. Also helps when using on a wet palette to keep everything moving nicely.