Sink Drain Stopper Won't Pop Up

I noticed just yesterday that the drain stopper in a fairly new bathroom sink (4 years old), had "fallen" down into the stopped position and could not be pushed back up as usual (by using the rod behind the faucet).

I pulled it up manually and found that it lifted the whole way out of the drain -- which it has not previously done. It had been locked into the drain. It now simply falls back down into the stopped position every time I place it back into the drain.

I've read the information at this short site, and studied the graphic of the drain stopper assembly. I think that I now understand the parts under the sink and how the drain stopper should work.
http://www.naturalhandyman.com/iip/infplumb/infpopup.html

However, the site explains how to get your stopper to lock back down tightly if it won't go down. I have the opposite problem: mine won't go up. From the information and picture provided, I can't figure out what happened. Can someone help me figure out what I should look for or try?

Just like the assembly pictured, my drain has a long plastic form that extends down into the drain, and which has a plastic "hook" at its lowest point. What should the hook latch onto? Is there a way to reengage it, or does the fact that it came loose on its own mean that something snapped? :confused1:

Thanks for any help you can provide!
Erin