Is it a carriage or segment shift model that you have? Was the machine stored for a long time? Is there evidence of rust on the machine? I've found it's fairly common for older standards that have sat unusded for many, many years to have seized components including the shift mechanism. If it only type uppercase letters than the segment (or carriage) is most seized in the shifted position. Can you move the shift key up and down on its own (I'm assuming of course that the shift lock isn't engaged)?

Ah, okay that makes more sense now. And I assume by "bank of keys" you mean the segment/type basket? I think on that particular model there are TWO very large springs, one on either side of the machine, that run from the back of the typewriter frame to the shift linkage. If they're missing it certainly won't work, and it's very possible that only one wouldn't be strong enough to pull the segment into place.