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.