Grey line on the screen when playing de video

The head unit that reads the tape is dirty or clogged, or the tape has moisture on it. Try fast forwarding to the end of the tape then rewinding back to the beginning, sometimes this clears the moisture. If that doesn't work try buying a head cleaner from an electronics store and follow the instructions to clean the head. if that doesn't work you need to take it somewhere where they fix cameras or send it in to canon to get fixed. If nothing else works it's most likely a bad head. I have the exact same cam (zr400) and the same thing happened to me but rather than send it in to canon, (who refused to honor their warranty saying it had major sand damage when it never even went outside) i opened it myself and replaced the head with the one from my older zr65 camcorder. It worked flawlessly. Hope this helps -J