What are you supposed to do if you lose your cell phone?

I’ve had a cell phone since I was 13 and I’ve never lost it, not once.

I think I finally broke the record today. I no longer have my iPhone. I really have no idea where I lost it, but it’s possible I left it in the library when I went there today, but other than that, I really have no idea. I had it at the beginning of the day in my pocket, now it’s no longer there.

So, I’m not certain if it really is gone, but assuming it were gone, what exactly are you supposed to do if you lose your cell phone?