What exactly is the problem with the display? Is it completely dark?
Several things can happen. The quickest checks, that don't need much testing gear:
- check if all those flat cables are seated properly
- check for any electronic components that look burnt , also, for dark brown or black spots/stains on the PCBs
- check if those tiny cathode filaments inside the VFD are not broken. Two photos below are from different models, but give you the idea
The left one shows intact filaments, the one on the right shows the topmost filament broken:
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