No clue really.
Maybe fill it with some warm water ~120-140F and it will spring back after a while? To help it a bit, try carefully to apply some pressure to it with a bicycle pump while the warm water is softening it.
For the future, prevent it from happening by not storing it with a bung in it. Just wrap a piece of plastic foil around the top, so it can breathe, yet it remains clean/sanitized. Even putting a rubber band around the plastic wrap, to keep in place, may cause it to suck a vacuum, which is what happened here.
On the other side, I wouldn't sweat it, this shape will not negatively influence its function, except you lost a quart or 2 of volume. Over time it may settle back to its old self.