Media Function Keys Randomly Disabled
Seems like we were able to find the problem through the comments. Chrome sometimes "hijacks" your media buttons for its own functions. A quick google search yielded the following result:


  You can set to the keyboard shortcuts to work only when Chrome is in focus:
  
  
  Open the Chrome app menu
  
  
  Select Tools > Extensions
  Click the ‘Keyboard Shortcuts’ link at the bottom of the page
  Find the Google Play Music section
  Change any specified media key options from ‘Global’ to ‘In Chrome’
  Fix Media Key Hijacking in Chrome
  
  
  
  
  Or you can disable/remove the app/extension from Chrome entirely:
  
  
  Open the Chrome app menu
  Select Tools > Extensions
  Find ‘Google Play Music’
  Uncheck the ‘Enabled’ box OR click the trash can to remove
  
  
  


Source: OMGChrome