Reason 1: The samples have been deleted from your [ProjectData] folder.
Reason 2: The [ProjectData] folder itself has been deleted.
Reason 3: The [ProjectData] folder has been moved to a different location on your hard drive, separate to the project's main file.
Please locate the [ProjectData] folder which corresponds to your project and ensure it is stored in the same directory as your main project file. The folder should include all of your project samples.
If files are missing, please restore from a back up.
If you haven’t backed up your computer, please visit Microsoft or Apple's website for guidance on setting up future backups with your Windows or Mac PC.