How to retrieve unclaimed BTC for wallets whose private key is lost How to retrieve unclaimed BTC for wallets whose private key is lost
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If you lose the private key for some address that contains bitcoins, then you will never be able to spend those bitcoins. The private key is required in order to spend, and there is no known way to work backwards to discover the private key for an account. 

If the private key is lost, the bitcoins become un-spendable.