Why can't I open Pokémon Go after playing in China?

I have been playing Pokémon Go since August 2016. I am currently level 18. During Christmas break, I visited China. I decided to see if Pokémon Go works there. I opened the app and found that I was able to log in. I played for a little while as I was waiting for dinner.

After that, however, I was unable to open my Pokémon Go app even after I came back to the US. I have deleted the app and reinstalled it, but still cannot open the app. When I click on the app, the Niantic logo appears for a few seconds but disappears. I did everything from closing all the apps to deleting and reinstalling the app. Now it is not working. How do I play again?

You may have been 'soft-banned' due to the substantial change in your GPS location:

Pokemon GO players have been noticing for a while that Niantic is “soft banning” (temporarily banning) players whose GPS action looks sketchy. If there’s a possibility a player might be spoofing their GPS, then they may find themselves soft-banned. Although Niantic hasn’t officially commented on this, the circumstantial evidence seems pretty strong.
_{- Stephanie Dube Dwilson - Heavy.com}

Niantic can't really tell the difference between someone spoofing their location (using a program to change where their GPS says they are), and someone that is legitimately traveling a substantial distance. a Reddit user had a similar situation traveling from Paris to Lyon.

There are a bunch of tips and rumors floating around about how to dismiss a soft-ban, but the only tried and true method is to check your location in Google Maps (with GPS turned on), ensure that it says that you are where you actually are, and then wait 2-4 hours for the soft ban to unlock itself.

If after a few hours you are still having issues I suggest contacting Niantic's Support