Where performance of a contract is impossible and its not either party's fault, the contract is said to be frustrated, allowing the parties to be released from further performance.
In order to prove the contract is frustrated you need to show that without default by either party a contractual obligation is incapable of being performed due to circumstances. You must also show the frustrating event was not caused by either party, it was not one where it was reasonably contemplated by either party and the occurrence was not caused by the party seeking to rely on the frustration.
A contract which is frustrated automatically comes to an end, however, any liabilities prior to the frustrating event may still be enforceable.