Sounds like someone stepped on a void. Voids are pretty common. If it is not crushed down, you can tap it and the sound will tell you the shape of it. Drill two 1/8" holes at opposite ends and inject thickened epoxy
using a syringe from Gudgeon Bros developed for that purpose. When epoxy
comes out the other hole it should be full. Let it harden, cover the two holes with a tiny drop of gelcoat
(epoxy has no UV resistance) and you're done.
If it's crushed down and badly broken, that's another story and a lot more work.