Thanks for asking the Apple Support Communities for help with your Pages app. Here is what you should do when it does not open, or opens then crashes.
I first recommend you force the app to close. This article will assist you: Force an app to close on your iPhone, iPad, or iPod touch
Once closed, restart the device using this article: Restart your iPhone, iPad, or iPod touch
Once your device is back up, try the app once again. If it is still closing, please try the steps in this article: If an app you installed unexpectedly quits, stops responding, or won’t open
Thanks and let us know if you need further assistance with this.