Pages - Files not opening

Hello, I really hope someone can help me with this issue. I recently upgraded to OS 10.14.1. It seems every time I upgrade my operating system, older pages files won't open. (Everything is updated.) When trying to open a file that I created last week, I get the message "The document ______ couldn't be opened". This is extremely frustrating. As a teacher, I need to be open files on my computer from the last few years...

Thanks in advance!