If you wrote it first then you own the copyright. Of course you have to be able to prove you wrote it first.
I had a similar thing happen to me, one of my domains expired and the people thought that bought it thought that entitled them to use the content i had on there. It doesn't.
I would recommend sending an email and pointing out the breach. If you look here
http://www.webproworld.com/viewtopic.php?t=57647 dutter gives a good example of the sort of tone you should take.
Good luck!