The lever in the base plate is attached by a slot-head screw to a simple lever on the other side, which latches on the edge of an elliptical hole in the inner bottom plate of the camera. It's a simple latch and as far as I can see it has no interaction with other controls.
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I thought it possible that the slot-head screw had come loose in your case. That would be a real pig. There's no way to access that screw through the lens opening, because of the pressure plate; and in any case the inner and outer bottom plates are being tightly held together by that lever.
However, if your lever simply won't move, that's a different problem. I hope others with lots of repair experience, like
@Andreas Thaler for example, will be able to suggest a course of action or a repairer. Failing that, I would be inclined to apply a very small amount of lighter fluid, and see if that frees it up. It's such a simple latch, it's hard to see that there can be much wrong.