How does that private key look like?
Does it begin with 5 and have ~51 characters?
I recommend you to go to
https://www.bitaddress.org/ choose tab "Wallet Details" and enter the private key you have.
Check if you can decode it there. If you can, try importing to a blockchain.info wallet what you get under "Private Key WIF".
If it still fails, let us know in what step and with what output.
Yes it start with a 5 an has 51 or 52 characters.
I went to bitadress.org put in private key in wallet details.
It says in private key wif all zero's