Game crashes whenever trying to load a world.

So, whenever I click to load up a new world, it should load it up, obviously, right?

Instead, now, once I click the button to load into the world, I load in for only a few seconds. Then, I crash. This goes for multiplayer too.

This happens whenever I load up a world. However, sometimes the world loads up nicely without problems. Later when I come back, it doesn't co-operate like so.