Generally because the wrong License Username is being entered: http://www.mediamonkey.com/support/inde ... is-invalid

Please put in a support request. There isn't anyone here who can help with licensing ..
Though I would say that *something* must have changed on your system. But again contact support (link at top of page)

My Media Monkey application has stopped excepting my License Key and now has gone back to the standard version. Brought this software 5/20/2015. I have now upgraded to windows 10.