When a Realm crashes, mob spawners in loaded chunks break. The manner in which they break is as follows:

• The miniature mob inside the cage to denote the spawner's type disappears
• The spawner does not spawn mobs, even in favorable conditions (i.e. darkness for monster spawners)

To fix the issue, a player can simply use a mob egg on the spawner set its type. The spawner will begin working normally until the next crash.

I don't have the resources to properly research this issue, but it has happened twice on a Realm I frequent. It could happen only as a result of a specific type of crash (the first occurrence happened in the middle of a series of banner placement-related crashes), but as of yet I cannot confirm this.