“My ex disappeared for a few years now she’s back and wants back support, can I take it to court and fight it, or will I have to pay it? I have no idea where her and my kids have even been for the past few years.”
Orders are orders. If there was an order for support before she left, then there may have been an arrearage accruing. There may be an argument that can be made based on not knowing where to send support amounts, frustration of parental rights or parental kidnapping, but you’re probably going to need an attorney for that. Going forward, yeah, your kids are entitled to be supported.