Your SOL
Reinvestigation the area and look for it again. If you can't find it contact your provider and report it stolen. If your on a contract then they should replace it sometimes for a fee. There is no way to track a cell with a dead battery and generally only the provider can locate it if someone decides to charge the thing. Some phones now have a tracking service which generally is a website that will show you a map location when it comes in service but this is a very new feature and not widely implemented just yet.