If the car has all original equipment then it is likely you have a Mk2 nav, becuase 4:3 screens were only used with Mk1 and Mk2 navs. The nav computer is the drive in the boot, what you have up front is just a keyboard and screen.

Show us a photo of the nav drive to confirm what you have.

Anyway, the 4:3 screens are known to fail when the screem reaches about 10 years old.

The screen is driven directly from the nav drive (if you don't have TV fitted) or from the TV tuner (if TV is fitted)

Easiest way to determine for sure what part has failled is to swap the nav unit (the drive) from your car to a friends car with known good working nav.

But I would be 90% sure it is the screen.

You may wish to take the opportunity to upgrade to a 16:9 wide screen