which version of word processor, endnote, and operating system.
This is usually because of a corruption in a field. First make a copy of the document and put in a safe place. Then Unformat citations to temporary or unformatted citations. Select all and remove field codes (select all and Ctrl+6 or ctrl+shift+F9) and delete any residual bibliography. Finally, reformat or update citations and bibliography (turning CWYW/autoformat back on). This usually repairs the document.