anything from dirt on the heads (a cleaning tape doesn't always work) to faulty heads, to a faulty pcb.... If your local camera repairer doesn't charge you for quotes, get a quote, but I would expect that the cost of the repair would exceed the replacement cost for the camera... Having said that it might be hard to find a mini dv camera these days, so if you need access to all your existing tapes it might be worthwhile fixing, although many broadcast video hire places will hire you a vcr that will accept dvc pro and mini dv tapes, and you could use this to convert all your existing tapes to a different format (either simply capture them to your computer or burn them to dvd)
Sorry for the bad news.