OS X Leopard Safari shows questionmark?
Pratika Rana is correct: your icon for Safari in the dock is no longer working. Remove it from the dock by dragging it out or by control-clocking and selecting 'remove from dock'


find Safari by opening finder and choosing Go -> Applications f of the menu. Double-click to open.
Safari, once it starts, will now have a new icon in your dock. Control-click its icon and select 'keep in dock' to make sure it doesn't disappear as you as you quit Safari.