In addition to what @Promidi has written, have there been any updates to your Windows installation since the last time you used MMS, or is there any Windows Update waiting to be installed or even partially installed? Have you tried rebooting your computer?
Is that warning coming up when trying to load mp3 files?
If so have you recently installed a different media player like VLC or uninstalled Windows Media Player as the program needs Windows Media player to work with mp3 files.