Why can't you edit the content from the front end?

From Joomla! Documentation

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 the file it was not closed properly.

If you were the person that last edited the article, log into the Backend of the site, go to the Content Manager and find the article in question. It will have a lock icon next to its title. Open the file and then Save it (or Cancel). When the content list is reloaded the lock should be gone.

Alternatively, someone with Super User privileges can use Global Checkin to check in 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 such as 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. Failure to do so will cause the locked condition as described above.