Krohne does not have a troubleshooting section in their converter/transmitter manual. Maybe Krohne thinks the stuff never breaks?
1) Is the display lit up and showing zero flow (when you know there is flow), or is the display dark (How do you know the meter has power) ?
2) Is the meter measuring correctly and providing a valid/good output, like the 4-20mA output, but not showing the correct value in the display?
Check the 4-20mA. What is the 4-20mA showing? Is it correct or incorrect?