If a performance is objectively impossible at the time of conclusion of a contract, no obligation arises. To render performance impossible, it is not sufficient that a particular party cannot perform, that is, subjective impossibility. The impossibility must be so serious that nobody can render the performance – that is, it must be objectively impossible. An example of impossible performance is where A agrees to sell his house to B, but unbeknown to them the house has already been destroyed by a fire. Initial impossibility of performance prevents a contract from arising at all.
If, after the conclusion of the contract, performance becomes objectively impossible without the fault of the debtor, as a result of an unavoidable and unforeseen event, this is known as supervening impossibility of performance, and the obligation to perform is also, as a general rule, extinguished. The requirements for supervening impossibility of performance are:

the performance must be objectively impossible; and
the impossibility must be unavoidable by a reasonable person.

If, after the conclusion of the contract, performance on either side becomes impossible owing to the fault of either the debtor or the creditor, the contract is not terminated, but the party who rendered the performance impossible is guilty of a breach of contract known as prevention of performance. It is not necessary that the performance should be objectively impossible in order for the breach to arise; subjective impossibility will suffice.   _{Your Comment}