I found this on a web discussion group:

This relates to the analog tach displays with integral digital hour counter that are made (sold as OEM) by VDO. If the tach still works but the LCD went blank, it is probably still incrementing the hour count but the display connection internally is bad. The LCD module is connected to the main circuit board via a flat ribbon cable. The problem is poor contact between the pads of the ribbon cable and the board receptacle. Lever up each end of the receptacle to release pressure on the cable and pull it out. Lightly clean the pads of the end of the ribbon cable with contact cleaner. You may also need to get a little contact cleaner inside the receptacle. Fully dry each with hairdryer before reconnecting. Be careful when doing that as you don't want to overheat the ribbon cable.

I edited the above info from the source thread which is here http://www.cruisersforum.com/forums/f14/hour-meters-have-stopped-working-on-both-4jh4ae-yanmar-displays-76479.html#post883908 I have this issue and was going to buy a new hour meter, but will try this first.