If you’re a MELPA user you might have heard that the URL for the site has changed. It’s easy to miss the significance of that for your Emacs configuration though. You have to change the package-archives list to reflect the new URL. That is, you have to change
(add-to-list 'package-archives
'("melpa" . "http://melpa.milkbox.net/packages/"))
to
(add-to-list 'package-archives
'("melpa" . "http://melpa.org/packages/"))
The old address is still working (it redirects to http://melpa.org) but you should change your configuration before it stops working.