The error C71 or Din Unlock will appear on the LCD of the recorder if the cable connecting the sound source has been disconnected during recording or if the sound source has been turned off. Verify that the connecting cable is properly connected and turn on the sound source.
If the error is still there, a power reset of the recorder may be required. For information on how to perform a power reset, check your manual. Manuals are posted on your model support page.