Contract Law - Trying to understand different aspects of contract law

Sarah entered into a contract with Peter to sell her prize bull Rufus for $20,000, with delivery from Sarah's farm to Peter within 30 days.

Fifteen days later during a rainstorm, a giant sinkhole suddenly appears, swallowing Rufus and Sarah's barn, neither of which are ever seen again.

What happens to the contract and why?

I am wanting to understand what happens when the item under contract no longer exist. What will happen?

Thank you for clarifying.