Give us more info! What do you have in front you, expecting to get a private key from? If you have an address (starting with 1, 3 or bc1q) that has thousands of bitcoins inside and you wanna find the private key for it, I will save you time saying it's impossible.
If OP has access to that wallet, he has access to all private keys in that wallet even he does not know it.
He can has options to dump wallet private keys or dump a private key of a specific public address. The same big condition is he has access to the wallet as if he can not open a wallet successfully, he is like a hacker that see a wallet but can not open it.
It reminds vital wallet backups for recovery in any case.