Website Hack/Breach

So my website is thelonetree.org . I was showing it to a “friend” and he seem interested. Well he went over to his laptop and did some things for about 15 – 55 minutes, then showed me something. He breached my site, adding a small black happy face under the footer. If you look closely, you can see it. In the morning I saw it (May 2) and now it is not there. When it was there, I checked the footer.php file and found no face. How could he have breached me, and have can I revert a change such as this? (P.S. ~ He said he cannot find the login page, but he might have done something with “View Source Information”) Any ideas?