When you place a password on a document and during that process tell Keychain to remember it, then as long as that document is located in your account on that Mac, Pages will ask Keychain to resolve that password, and the document just opens when you access it. Move that document out of your account or off of your Mac and there won't be any Keychain to help Pages resolve the password.
If that document that won't open remains in the same account on your Mac and it was "remembered" in Keychain at the time of the password creation, then provided you have admin privileges, you may be able to get the password revealed.
Click on your Desktop and press shift+cmd+U to open the Utilities folder. Double-click the Keychain Access application and select Open Keychain Access. You will be presented with a default long list of entries. Scroll down looking for the name of your Pages document that you have no access to. If you don't see it, then Quit Keychain Access and there is no hope of finding the password.
If you do see a line in Keychain access with the name of the Pages document, select the line, right-click on it, and choose Get Info. A new dialog will appear and at the bottom of it, you will see an unchecked box with the name Show password. Select it. This will open a password challenge that wants your admin password. The blue Allow button will light and you press return. Now your document password will be shown in that Show password field.