L322 electrical gremilns?

The inlaws boss has an 04 plate. His sat nav screen went off, and he was told that 1st thing to go off when battery flat. So he changed the battery, and still no sat nav. The radio, and cd changer works ok, but nothingg shows up on screen. Could changing the battery cause this, or more likely just a problem with the screen? Is there any way to test it?

Do a search, I'm sure I read that there is a problem with water getting into the unit which is housed behind the nearside boot trim.