misto sprayer malfunction
How apart did you take it? There are 2 o-rings, 1 inside the spray mechanism and 1 on the outside of the larger plastic tube where it connects to the oil container. Losing either of these would pose a problem.

The nozzle on a Misto does not push down very far at all, maybe 1 mm at most. When not under pressure it is hard to detect any up or down movement.

It is possible if you pumped it up, tried to spray and nothing came out, that it is clogged. The answers to this question may help How to clean a clogged misto oil sprayer/spritzer.