pages

Do two things:

1. When Pages crashes or hangs, the next launch of it should involve you holding the shift key as you launch Pages. This prevents it from using its saved last application state again. Try opening any Pages document afterward.
2. Reboot your Mac into Safe Boot mode by holding the Shift key until you see a progress bar. This will be much slower than an ordinary boot as it is doing some maintenance things for you. Again, launch Pages as in [1], and try opening a document, then reboot normally.
   1. Upon rebooting normally, try Pages again to see if it allows you to open a document.

In the future, instead of typing just Pages in the subject line, you will get better help response by briefly stating the problem (e.g. Pages v10.3.5 Unexpectedly Closed).