You can easily diagnose from where comes the leak :

- Was your computer on when the coins have been sent ?
- Do you have an anti-virus software or firewall ? Was it up to date ?
- Did you have the same password on your wallet and on any internet website/service ?
- Did you enter your password anywhere ?

If I'm right, now your money is here:
https://blockchain.info/address/1K44FRM82amtFBNY6kcJaMb5uUMKDtpoKN
12.47 BTC

And 3.64 BTC was spent here:
https://blockchain.info/address/1EfgpbHDJYvm4VC21WomUTEpLNTrjyV5Cz

Among with 50 BTC going here:
https://blockchain.info/address/1NJghHFxp6GjecKgMtam3mVnued7qFRxQ1
Can we check wether this address is on Bittrex or any other exchange ?

I don't know if that's relevant.. but my Bittrex BTC address also starts with "1N"
coincidence ?

Best regards