The most common cause of this problem is the item is "checked out." Either the item is being edited by another user or (more commonly) that the last time someone opened (edited) the file it was not closed properly (or saved).
If you were the person that last edited the article, log into the back end of the site, go to the content manager and find the article in question. It will have a lock next to its name. Open the file and then save it (or cancel). When the content list is reloaded the lock should be gone.
Alternatively, someone with superadministrator privileges can use Global Checkin to checkin all checked out items. If using this option, make sure that no one is actively editing a file or their work may be lost.
It is possible to check in your own items that you may have accidentally locked if a link to checkin is available. This may be called something else. You can add this link to a menu by creating a URL link like this: index.php?option=com_user&task=CheckIn
Note: It is important that whenever you open an article for editing you should close it using either "save" (floppy icon) or "cancel" (red x icon) when finished with your edits. Failure to do so will cause this lock as described above