Check your Starter, Like what I did in the First Couple minutes of this video. I put the Positive Jumper on the Starter Terminal ,and then Touch the Ground cable to the Starter Frame to make the Circuit ..That way you don`t arc out and Burn your Threaded Terminal.
Along with Stony`s Suggestion ,There`s also a Safety switch for the PTO.Check that switch too. If the wire is Broken Between either of those switches you won`t get power through that circuit to your solenoid.It`s most likely going to be something silly/Simple like that..Battery has a Charge, etc. Do you have a Test Light ? That will go a long way in Helping to figure this problem out Quickly.. If you don`t have 1,, Make one up with a 12v.Light Bulb from a Car with a Couple wires Soldered to it.That way you can Actually "see" Where the Problem is by Probing along the Circuit till you Stumble into the Problem.. Right now, We`re just Guessing. And we can keep Guessing for a LONG time..