What happens now after I was found in contempt for not paying half of my taxes stipulated in my separation agreement?

You must find a way to pay. Try to get on a payment plan with the IRS, get a second job, take a loan, whatever it takes. Otherwise you will eventually end up in jail and owing even more money (as ongoing contempt is going to result in more attorney's fees . . . not only for you, but you will owe your ex's fees as well).