Hey Colin, sorry for the delayed response.
If your still needing assistance let me know, before assuming the Nav system has failed I want to see if this is a possible firmware issue that requires a firmware upgrade.
You need to disconnect either the sat/nav unit or the battery for 5 minutes then re-connect.
IF the system works normally for a short period of time after doing this and then reverts back to the blank screen, then it's likely just a firmware issue and you can have the dealer upgrade the software to get it going again.
Colin, Thanks for clarifying with more details.
Start by checking the fuses by the drivers knee panel. If they check out, then unfortunately we still need to perform a HARD RESET, before going further.
Thanks, ***** ***** you then.