okay, take the top off and check the wires leading to the 1/4 output on the jack. They might be disconnected somehow - if they are, all you have to do is resolder them back on. not hard, should only be 2 leads there.
If that doesn't fix your problem, it could be the preamplifier. Your only option in this case, unfortunately, is to replace the preamp, that'll probably run you anywhere from 100-400 dollars, depending on your model of rhodes.
The key not playing is actually a pretty easy fix. that's a broken tine - when you open the top, check the keys that aren't playing. when you look at the row of tines, you'll notice that the tines of the keys that aren't playing are broken off so that the hammer won't hit them. Just get replacement tines for those keys - i got a big box full, because i break them a lot (i play hard sometimes). Long tines can also be trimmed with wire cutters to be short tines in a pinch, but that'll effect the timbre of the sound somewhat.
hope i helped some.