panasonic nv hd600 won't eject and displays F004?

After successfully playing a video, I try to rewind, which doesn't seem to complete, and I can't eject the cassette. Even the power switch just toggles between two modes, but not 'on' and 'off', then when I press and hold 'eject', there is some rewinding sound for a few seconds before the display shows F004 and there is silence. I guess it's now seriously broken in some way, probably too difficult to repair? But how can I retrieve the cassette?