In the first third of the Halo 4 campaign mission “Reclaimer,” around the start of Rally Point Bravo, the Covenant’s Lich appears overhead. Sometimes after completing this encounter, the Lich will fail to explode and the remainder of the mission will unload. This unloaded area appears as a large void and blocks all progress in the mission. 

To progress past this point in the mission, you can try restarting the mission from the beginning or restarting from Rally Point Bravo. Restarting from the previous checkpoint of your current progress will not resolve the issue.