When an admin clicks on a content page it gets locked to prevent others from editing it at the same time.

It becomes unlocked once the admin clicks "Cancel" or "Save".

However, if the admin closes the browser or leaves the page some other way, the file will remain locked until the admin in question returns to the content page and clicks "Save " or "Cancel".

Alternatively:

To unlock a page, logon as SuperAdmin and click System --> Global Checkin (at the end of the menu bar) this will unlock any files.