Internet

There is a federal law (18 U.S.C. §2258A) which requires "electronic service providers" such as Google to report apparent child pornography to the National Center for Missing & Exploited Children's (NCMEC) CyberTipline. This is a special exception to the federal privacy law governing ESPs which is otherwise quite strict; ordinarily, companies such as Google are prohibited by law from sharing information regarding your private use of the Web with government agencies. More than likely Google sent this notice out of an abundance of caution to ensure that they complied with the law.

It's entirely possible that nothing will happen if this was an isolated incident. One question to consider is whether anyone else had access to the same computer that you use to browse the Web. (For example, do you have any teenagers in the house?) If you do receive any further communications, either from Google or from law enforcement investigators, consult a knowledgeable criminal defense attorney in your state.