Hello, Sorry you're having problems. In most cases when any electronics fail to operate after being in storage for any length of time (and 15 years is quite a long time) the most common issue to occur are dried up power supply filter capacitors in the organ. They are very similar to batteries, in that they use electrolytic fluid inside to separate the positive and negative plates, and prevent them from shorting together.
That said, it is generally recommended to have a "Cap job" done on any electronics that are 15 years old (let alone used and then stored for 15 years). Since the fluid in the gaps of those capacitors dries out, it will create a short circuit when the unit is powered up and a voltage loss to many of the circuits that are integral to sound.
While this organ was close to $10K when new (between 1967-1973), it's current re-sale value is between $1,200-$2,500 on average. With the cost of having a cap job done (and possible replacement of any carbon composition resistors which have drifted out of spec), being between $250-$350 parts and labor, it is cost effective to have the organ repaired.
You can also try using the link below to locate a Hammond organ repair service in your area (or hopefully close to it):
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