Hi Hun
If your play doh's gone hard (because it's been left out) then I don't think there's much you can do.
This is the homemade recipe my MIL uses though. It's great - although it has a little bit of a funny smell, but you can combat that with a few drops of vanilla extract.
1 c. flour
1 c. boiling water
2 tbsp. cream of tartar
1/2 c. salt
1 tbsp. oil
Food coloring
Mix and knead together
love