Problem: A contributor closed the browser window before closing the article and now it has a lock. Others can't get in.

Solution: This is a common problem when you have multiple authors for your articles. If you can get a hold of that contributor, have them open up the article (they can if they created it) then Close out of the article. 

If you can't get a hold of them, find someone with Super Administrator rights. They can get into any article. They can also select the Check In button at the top of the Article Manager and check in all locked articles (they must select them from the list using the check mark).