Likely to be one of two problems;
i) deterioration of the connection of the ribbon from the LCD. These are ususally bonded to the pcb using a special (and very expensive) conductive tape made by 3M, which conducts through point to point, but not sideways. It's very difficult to repair successfully, and requires some heat and a way of clamping the joint. Carefully clamping and re-heating the existing joint is sometimes successful. Specialist job really.
or...
ii) The polarising filter in the display has become damaged by sunlight. You can check this by holding a polarising filter sheet in front of the display, and rotate it to see if the digits magically appear. If it works, you can disassemble the unit and affix a piece of the filter to the front of the lcd display, cut at the correct rotational angle. You can buy the sheet on eBay, or a good source is scrap LCD displays from laptops - if you pull one apart, there's a polarising sheet inside.
I would go for the latter as being most likely where you are, and easier to check anyway.