Lost photos in iPhoto library

My Mac OS X 10.4.11 is owned by my sister. After the home icon, it always said her name. Today, I changed the the of the folder to Home. After I did that, all the photos I had put into the Iphoto library are GONE! How do I get them back and where are they?? If I try to change the name of home back to my sister's name, it says that name is already in use. HELP!