What to do if I lost my iPhone?

I lost my iPhone 5s this morning(probably dropped it when leaving my car)

What should I do? I tried locating it through icloud, but it says my devices are down so I probably put it on airplane mode.

Any advice/help will be greatly appreciated.

You call your cell phone provider, have them disable the phone's ESN or IMEI. (whichever your phone has), so anyone finding can't activate it on any carrier, and they may return it if they can't use it or sell it.