I'm guessing you are using iCloud for the documents. If not, most of this won't apply.
First, if you have access to a computer go to icloud.com and sign in. See if your document is there. If so, make a backup (download) of everything important. Probably not necessary, but no reason to make chances.
First thing to try. Go to Settings > Pages and turn off Use iCloud. Then see if you can get the app to run. If it does, then try turning iCloud back on and (hopefully) it will sync and work again.
Second, delete and reinstall Pages. Don't do this unless you've got copies of your documents.
If none of this works, or you can't do it for some reason, like not being able to back up your documents, add some more details and maybe someone can come up with a better idea.
When this is over, I recommend getting a good cloud service like Box or DropBox and making a habit of saving copies of your work there. Like any computer, iPads and apps can fail. Never trust your important stuff to just one place. Three is best, with one of them being unrelated to the other two both in physical and logical space.
