Whether you have ownership would need to be stated in the TOS of the site.
You could go to court over a breach of contract (if that is the case) but do you really want to spend thousands of dollars in legal fees to accomplish this goal?
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.
As noted by Mr. Jaffe, your rights depend on the TOS. That said, absent some extraordinary and frankly incredibly stupid terms, you don't legally own anything in an account. An account is essentially a license to use the services. That means you just have limited rights and a license, by definition, does not confer ownership.
You, as always, can sue anyone for anything. The question is if you will win and based on your limited facts, I would find that doubtful.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
While my colleagues are, of course, correct that the terms of service control the ownership of the account, along with any content you posted, it has been my experience that many website operators do not properly implement their legal terms, in order to form a binding contract with their users. The shoe company, Zappo's, learned the hard way recently, when a court invalidated its online terms due to failure to properly gain assent of its website users. You may have an argument on ownership if the forum did not legally implement its online terms. A lawyer will be able to answer that question for you.
Disclaimer: The foregoing does not constitute legal advice or form an attorney/client relationship. Please contact an attorney for formal legal advice on any specific matter.