If a party to a contract cannot complete that contract because of some intervening event then the contract still stands. However, discharge of the contract may be allowed in some extenuating circumstances or frustration. For example, performance would be an illegal act but one that was not illegal at the date of contract.
Similarly, frustration under a lease can only apply in some rare circumstances. For example, in Taylor v Caldwell (1863) 3 B&S 826 the use of the Surrey Gardens and music hall was booked for 4 days but before the first day the hall was destroyed by fire. It was held that the contract was frustrated.