What can happen when a ganglion cyst pops?

The fluid: Drains and it shrinks. Just keep area clean.

Pain, may resolve: You may have some mild pain, and the cyst may simply resolve. For example, ganglionic cysts in the wrist for example, used to be called bible cysts, as people would use a large book, like a bible to "hit them and rupture them." this of course is not recommended to do nowadays. Best wishes.

Nothing: Particularly serious. Many are without symptoms and regress over time. Some are painful but still regress over time. Aspiration of painful ganglion cysts provides some temporary relief, but usually will recur. Even surgical excision doesn't guarantee they won't recur.