This error usually means that the drive containing the content for the instrument was removed or a file was modified while the instrument was open. If you know what happened, please close your DAW, rectify the problem and then relaunch the DAW.
If you do not know what happened and the problem persists, please contact our support department by clicking on the chat icon in the bottom right of your screen. If you don't see the chat icon, click on the "help" icon with the question mark and submit a ticket.