well there are apps out there that allow tracking your phone in case of such an emergency but my guess is that you didnt install one.
the easiest thing is to call your phone. if you left it out in public someone probably already found it and might answer. if you're lucky the person can get it back to you.
if your gps is on maybe your provider can locate it, if someone makes a call on the phone they might be able to triangulate it (if they put forth that sort of effort!)
if your phone battery died theres not much that can be done. and seeing as how batteries die fast on androids that might be the case.
if someone stole your phone your provider can always brick it to prevent it being used (though you'd have to buy a new phone at full cost probably)