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In my experience, this sort of thing may respond to some simple 'fixes' such as the following - which are not sequential, by the way:
* Rebooting your computer if your rarely do that. (It astonishes me, sometimes, what this will set right.)
* Closing all your browser windows and opening a new one
* Possibly clearing cache and cookies.
* Not going to AdWords via a book mark - but by opening a new browser window and getting to the login page by typing in the URL
* Trying a different browser.
* Updating your current browser to the most recent version if you are using an older version.
* Trying a different computer if you have one available to you.