Sounds like the seals/0-rings just disintegrated
The seals and o-rings in a toilet pump don't last forever...they wear out from rubbing against the inside the of cylinder. They wear out a lot faster if the pump isn't kept well lubricated. If not replaced on 5-6 yr schedule as preventive maintenance, they will eventually just disintegrate. If your toilet is much older than that, I'm guessing that's what's happened to at least one--maybe the last surviving one--of the rubber parts in your toilet. Installing a rebuild kit should cure it, but whether it's worth doing instead of just replacing the whole toilet depends on the make/model/age of the toilet...'cuz if it needs more than that, you could end up spending more to fix it than a new one costs.