You say you have "had ownership" -- but if you don't hold the title, you are not the owner. If your friend told you from the outset that her father was the record owner, you probably have little more than a constructive lease of the vehicle -- you paid her for its use from the date she gave the vehicle to you, since you knew or should have known she had no authority to sell.
If your friend didn't tell you that her father was registered owner, you may have a fraud claim against her (if you didn't know she wasn't authorized to sell). Did you have any written agreement with your friend for the vehicle? The terms of the agreement, written or oral, will control what actions you might be able to take.
Finally, I'd guess you don't have the required insurance on the vehicle, since you have no proof of ownership. And since you don't have the title, you're also driving without the registration information required. Have you considered what could happen to you if you're pulled over by a police officer -- or if you're in an accident?
Bottom line: if the owner didn't sign the title over to you, you don't own the vehicle. And you could also be exposing yourself to significant civil, and potential criminal, liability (if she hasn't been able to reach her father, what would prevent him from reporting the vehicle as stolen, or filing charges against you if an accident were to occur?).
You don't say where the friend's father is located or why she hasn't been able to get in touch with him for six months. If she's telling you the truth, and if the father is MIA, you could hire someone to trace his whereabouts so that you could contact him directly. If you know where he's located, you could contact him -- but there again, since apparently you made the deal with his daughter, he's under no obligation to honor it and he could demand that you return the car immediately.
I'd recommend you get yourself to a lawyer sooner, rather than later, to sort out this mess.
Of course, as with all of my online answers, my advice is limited by the brevity of your question and the facts provided. Additional information would be required to provide definitive legal advice, so this answer isn't intended to, and does not, create an attorney-client relationship.