You can't fix it. You can try to patch it up with some sort of epoxy or other sealant, but the reality is that a pool is nothing more than a big bucket, and if there's a crack in it then water will eventually leak out and you'll have to fix it or replace the entire thing. It's best not to wait until there's a problem, but rather to be proactive and regularly check for cracks and other damage so that you can address them before they become an issue.

You can fix it by filling the crack with a waterproof sealant or epoxy. Once the sealant or epoxy dries, you can then fill the pool with water.