Mini DV tapes store video information in a digital format, meaning that it is highly unlikely that a physical recording issue is at fault for the blanking. Additionally, knowing that in-camera menus are not affected informs us that the display and associated circuitry is not at fault either.
There are a few distinct possibilities left.
# '''Sensor Damage''' - Most Mini DV camcorders feature a IEEE 1394 or Firewire port. You can use this port and a computer to download or view the video files directly, and determine if the grey bars are actually being recorded.
# '''Decoding Fault''' - Friends or family may have another MiniDV recorder. Pop a tape from your camera into it and see if it plays correctly, if not, it will be an issue with your camera.
In the end, MiniDV is an obsolete technology, which leads me to believe a repair will potentially cost more than a modern replacement device. Best of luck!