I have to agree with atm424 on this.... when I experienced this behavior, my shrimp were poisoned by the air. In at least one of those cases, a neighbor had bug bombed their apartment.
If you can move your shrimp out of the tank for now into another one, or even a container, I would recommend doing this and just doing large water changes on their current tank. Even if you can't move them, I would still recommend large water changes.
Could be CO2 related, anaerobic bacteria related, or even fertilizer or bacteria related. Something that's been added to the tank if it's not an "air pollutant". (only affecting one tank)