Trying to get property back
I moved from chicago to New Hampshire a week before christmas, and couldn't take my computer with me. I asked my dad to hold onto it for me until I could find a plan to have it sent to me, since I was short on money at the time.
My dads at-the-time girlfriend hooked up the computer and started using it, even upgraded the video card, all without asking permission or without me even knowing it had taken place. Now they have broken up, and my dad left the computer at her house with her using it.
I had originally called her and she told me what happened, and how she was using it, and we were talking on good terms trying to find a solution. She said she'll either put the video card back in and send me the computer, or send me around 300 dollars for the computer to help me with buying a new one, which I was fine with. But after that phone call, she never answered my calls again for 3 weeks until the present, hasn't answered texts or voicemails. I've even contacted her son who I was good friends with, and he told me he told her, and she just doesn't care.
So now in a couple days, I'm moving back, and I was wondering what the best course of action would be if she doesn't return the computer to me or give me the 300 dollars.
I don't really have proof that it's mine, except that my dad knows it was mine and he let her use it, and her son knows its mine. What can I do?