Answer:
It could be several issues that affect the sign... It could be a burned gto wire going from the transformer to the tube, or it could be a burnt up transformer, or a broken neon tube.
Typically, the neon tube is removed from the sign. If it is a red gas tube, it can be repaired by bending up the missing section, and welding it with fire back together. Then it must be pumped (bombarded)
If the tube is green, blue, white, yellow, or any other color that uses Mercury to aid its light output when originally lit, then great care must be taken. Vaporizing mercury is very toxic to the health of neon workers and this type of tube needs to be made new, with the old tube being properly disposed. I have fixed a few mercury tubes, but I do not advise it to anyone. If the state finds out where mercury tubes are being repaired, they can come close you down if they find loose mercury or contaminated premises, or worse... do something that affects the title to your property!!