No need to panic. Nail polish thinner is your friend. Even the most dried up crusty looking nail polish can be resurrected with some thinner, and patience.
For polish that is just a little goopy or thick, just a few drops will do.
I use both Seche Restore, and Beauty Secrets thinner with great results. Add a few drops, roll the bottle around, perhaps tip it right upside down for a few minutes, or if you are not wearing it immediately go ahead and shake it up.
Test the consistency and add more as needed.
Some polishes, like Crackles and Seche Vite or Poche will need a few drops added every time you use it.
If you happen to add too much, just leave the bottle open for a few minutes and test again.
Thinner is simply the chemicals that have evaporated. Over time, some nail polishes will evaporate and thicken, even if you never open them. The chemicals still leach out even if you have the tops on tight. If you have ever stored your polish in a plastic bin with a snap on lid, you have undoubtedly noticed the distinct smell of polish fumes when you open the lid. Those are the chemicals that are evaporating.
Just please do not used acetone or nail polish remover. This is the quickest way to turn your beauties into unusable muddy messes.
If you have a polish that is separated, just shake it up. No need to toss it.
If you have a polish that is completely dried up and hard, it can be revived. Add a decent amount of thinner, and let it sit over night. Give it a shake in the morning to see how it is doing, and if it's still a clump at the bottom, use a toothpick or a chopstick to poke the polish and allow the thinner to seep into it. The surface of the dried up polish will have softened enough over night to allow you to manually mix it up a bit, then add more thinner as needed and repeat. When you hear the glorious clinking of the steel balls against the sides of the bottle, then you know you are likely very close to, if not at, a usable consistency.
Do keep in mind that Seche Vite is not a B3Free polish, and neither is their thinner. If you are adding Seche Restore to a B3F polish, it is no longer B3F.