When you are editing an article, if you ever click on your back button on your browser instead of clicking 'Close' or 'Save', this will lock your article. You'll see a padlock icon next to the article and you may not be able to open it anymore.

You shouldn't use the back button to close an article, but rather use your 'Save' or 'Close' buttons. To remove the lock on an article, simply go to Tools>Global Checkin.