If you can prove you wrote it (in the US) it's yours. If someone else has copyrighted it and you can prove them wrong then the courts MAY even order damages -- but that's a slippery road because if the other party can show "reasonable care" in searching for the author, well....

Intellectual property only goes into public domain if its copyright has expired or the property has been "abandoned." And let's not get into the "fair use" conflict....