Well, first off just accept that it is gone. That is the first step.
Unless you can somehow trace down someone within a reasonable amount of time before they take off or there is no time left, just let it go and remember that there are more important things in life. That seems to help.
I still remember it very clearly. I was at xc camp, playing frisbee and soccer and stuff in my free time. I set my phone and my bottle down next to this one building and when I went back… no more phone. BUT THEY DIDN’T TAKE MY BOTTLE. That was the important thing, anyways. First thing I did was look around because I do stupid things like this sometimes. While I silently and hopefully looked around to confirm my suspicion, I took up my fate and started my plan of action. I knew that I was able to remotely disable the phone through the android services so I got another kid’s phone, called up the at&t service line, and disable that crap. Afterwards I let my parents know, let myself know again, and continued my camp unaffected. Except that I couldn’t blog. That was the real struggle.
The point is: It is a safe and smart idea to back up personal files in the cloud so that when crap like this happens you don’t lose important pictures or text memos or anything that has any sentimental value because you can replace the Instagram app or just reinstall Snapchat, but you can’t recover all of the amazing photos you took with your friends and family. At least the person has a brand new phone he/she will never be able to sell (except illegally). I hope that at least they are happy.