An extinction event. I love that there is a list on this in wikipedia:

• super volcanoes erupting and filling the skies with ash
• over population
• whatever is going on with the bees
• nuclear war or societal collapse
• climate change
• destructive asteroid/meteor
• global pandemic
• black holes
• unfriendly extraterrestrial life
• ecological disaster or water shortage
• gulf stream shut down
• artificial intelligencia take over