The most probable reason is that you or someone using your computer has deleted it, maybe without being aware of it. Or, it might not be deleted, but just hidden, so you should search your files.
Assuming this is in reference to Internet Explorer Favorites, there could be a few reasons. Do a Windows file search for "all files and folders" for "Favorites", or just look in the path C:\WINDOWS\Favorites folder. There should be a long list of files there.
You must have acciddently erased all browsing history from the IE. OR you must have erased the favourites from the Documents and settings in folder C. Try not to surf into the folder C or any partition where windows is installed.
Windows xp is considered old now and you could be losing data from your hard drive. The hard drive is not working as well as it should and programs disappear. Or you can set your computer to an earlier time this might bring it back.
