What if someone registers copyright on a piece of work that I created in the first place but did not register?

First off, I'm not a lawyer; you should consult a lawyer for actual legal advice.

While copyright law varies from country to country, here in the U.S. any original work you create de facto is copyrighted and your property, even if you don't register it with the copyright office.

From the U.S. Copyright Office FAQ [http://copyright.gov/help/faq/faq-general.html#mywork]:

When is my work protected? Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.

Do I have to register with your office to be protected? No. In general, registration is voluntary. Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work. See Circular 1, Copyright Basics, section “Copyright Registration.”

Why should I register my work if copyright protection is automatic? Registration is recommended for a number of reasons. Many choose to register their works because they wish to have the facts of their copyright on the public record and have a certificate of registration. Registered works may be eligible for statutory damages and attorney's fees in successful litigation. Finally, if registration occurs within 5 years of publication, it is considered prima facie evidence in a court of law. See Circular 1, Copyright Basics, section “Copyright Registration” and Circular 38b, Highlights of Copyright Amendments Contained in the Uruguay Round Agreements Act (URAA), on non-U.S. works.

If someone is using your work without permission, I would first contact them and ask them to take it down; maybe they didn't know the licensing constraints on the work.

Depending on the medium, there are different routes of action if they fail to aquiesce to your request. If it's online content you can submit a DCMA takedown notice. If it's another medium consult a lawyer. Either way, consult a laywer.