I'm not saying what you should do, and I'm not saying you should do what I'd do. I'm just saying what I'd do.
First, is there any chance they didn't even use the articles because they didn't accept/pay for them?
This may not actually all the potential issues, or cover all the right legal aspects of things; but if it were me I'd just use my articles and claim my ownership to them, since an agreement that said you'd be paid for the articles might be said to be null and void if no money was paid. It may not help you much if the articles are already online and would be there to compete with themselves in search, but my thinking is that if I didn't get the money agreed upon for the articles the articles would still be mine; and I'd put them out there with my name anywhere I could put them out there - even if I didn't get money from them. Let whoever stole the articles from you (and not paying for something is stealing, or at least conning) worry about how well they do, or whether they're all over the Internet with your name as author.
And, if it was one of these deals they you signed something to say you understood you may not be paid unless they use your articles (or that kind of thing - the kind of thing that essentially said you understood you may not be paid for one reason or another)... I'd still use my own articles wherever I could, with my name. I'm only guessing, but I'd bet there's a good chance they aren't about to get a lawyer to come after you. They may try to claim that "their" articles (if they're already published) were stolen by you (and that could cause you problems if you have a Google account). Then again, you could take your chances with that and if it becomes an issue then explain that you weren't paid for the articles you wrote.
The only thing that might be tricky is if you signed something that said something like, "I understand that by submitting these articles I am signing over ownership of them to xxxx upon submission, whether or not I'm paid for them". If that's the case you can either go with having been essentially conned into handing them over, or else you could choose whether or not to risk having these people make an issue out of it if you go ahead and use your articles. Keep in mind, though, that for people who con writers into sending them articles it's pretty easy to just come by more articles the same way; and lawyers and complaints take time and/or money.