Confirm that you are not logged in with an account assigned the Student Worker Restricted role. If you are not, clear your browser's cache and WorldCat.org cookies. If you've accidentally downloaded a corrupted page, it will clear it out and allow you to get the full page so the button will reappear. If both of those fail, contact OCLC Support with your symbol, WorldShare username, and the troubleshooting steps you have tried before now.
There is more information on the WorldShare Circulation roles used to perform checkout.