Hard to imagine this scenario is as-OP presented it. If I am a 2-day-old FBO owner, I'm probably not going to start out kicking my customers in the bonch and extorting their aircraft.
Then again, FBO shenanigans surprise me now and again with the depths they'll plumb.
If I were in OP's shoes, and the story is true, there is a source of leverage somewhere to be found and applied. If this is an airport of any size, then this FBO pays rent to a Master Leaseholder or to Airport Management. Assuming OP is transient, and has no "hey I buy gas here all the time" story to lean on, then maybe a word with the landlord or airport manager to request his help arbitrating is warranted. Basically, you need friends who can look the FBO in the eye.
If this is one of those Podunk 'Airport n Gas n Such' places that seem to litter the midwest, he's probably screwed, and Bubba the new FBO manager / Airport Manager / Lineman / A&P is going to get paid somehow. He's probably cousins of the former FBO manager. Maybe even double cousins. Probably related to the judge, the sheriff, and the only lawyer in town too. Best bet? Write the plane off as lost, buy another one, and stop leaving it at little ****burg airports with no services and 2 weeks' free parking.
It's not a classy business. I don't miss running one. Nobody ever left a plane on my ramp for 2 weeks, but I'd have likely just towed it onto the city tiedowns and made it their problem, rather than hide the thing and hope to score cash money millions.
$0.02.