Try Ctrl clicking or right clicking the top area of the web browser window and if "customise" is an option then you can use that to restore the defaults.
If you mean the Top Sites window that shows if you don't set another page as your Home Page, then we need more info as it sounds a bit odd .... my first advice would restore your Google bar and then just use it to go to any site you frequent and it should by the very nature of "Top Sites", be added back there ....
You can just drag a site's URL to the Top Sites icon in the Bookmarks Toolbar to add it there.