Why can't I open the sliding passenger door from the inside?

What does the inside push button do? Is there resistance like normal or does it feel like it's not attached to anything?

Didn't really answer the question. If it has no resistance at all, there is a cable that runs from it to the mechanism that may be disconnected but that is not common. More likely the button broke; it's just plastic.

What IS more common is for a linkage to become disconnected from the mechanism. The ends look like a threaded wood screw that just snaps into a spring-metal clip, then "magic" holds it in. After slamming the door enough times the magic isn't strong enough to hold it and the linkage pops out. Depending on which linkage pops out, the door may not open from the outside, the inside, or both.

You'll need to remove the trim panel to see if a link is disconnected. If it is, they are spring-loaded to sit in the proper position so you just have to snap it back in place. I add a small dab of silicone gasket sealer to the clip so the link doesn't pop out again.