What does your private key start with? If it doesn't start with 5, L or K, you are out of luck. It isn't a private key.
I never used coinkite so I couldn't know, but if ranochigo says the truth, then there is no possibility to retrieve you money from there, but I would still suggest to contact their support and maybe they will review your case and give back the coins, because 0.5 is a big sum.
You need to know all account details to do this, they surely won't give them back if you cannot answer them for a every single question.
Obviously they shouldn't ask you about password but about other sensitive data, I wonder if you will be able to have them back.