If you discover your own work being used or shared unexpectedly, you may first wish to consider the possibility that someone else is making fair use of your work. However, copyright infringement (sometimes unintentional, sometimes malicious) is not uncommon. If your work is being infringed, there are a number of steps you can take.
Consult an attorney. An attorney can take direct action on your behalf, and attorneys who have special expertise in copyright may have a lot of potential solutions for you. However, attorneys can be expensive, and there are some self-advocacy steps you can take as well.
Ask them to stop. It is almost always a bad idea for a non-lawyer to send a threatening "cease and desist" copyright letter - even some attorneys make mistakes with legal threats around copyright issues, if they are not specialists. However, a polite explanation of your objection and request to stop the problematic practice can often work wonders. And if the polite letter doesn't work, you can follow up with other measures.
DMCA takedown. Many websites that host user-generated content will remove or disable problematic content if you contact their DMCA (Digital Millenium Copyright Act) agent. However, under the same rules, if the individual who uploaded the your material responds asking for their content to be restored, many sites will (appropriately - this is how the law is supposed to work) restore it. At that point, you would do well to contact an attorney with experience with copyright matters.
Don't wait too long! There is a statute of limitations on copyright claims - if you wait more than three years after you find out about the potentially-infringing use, you may not be able to take legal action. (In some jurisdictions, the statute of limitations is understood to begin at the time of infringement, not the time you find out about it! Cases in these jurisdictions expire even faster!)
However, don't rush in too quickly! It is easy to make very sweeping arguments and threats when you feel your rights have been violated, but this rarely plays out well in the long term. You may find that some infringing uses are not worth your time to pursue. Note that you cannot lose your copyright ownership by failing to police your works. People are sometimes confused about this, because you can lose trademarks through neglect, but you continue to own your copyright for the full term, or until you transfer it away.