Whoa,
somehow my stairs were not connected anymore. I lost the connection between my first and the second level. How is that possible to happen? Can I have changed a up/down stairway to a just down stairway? Because I had a dwarf trapped in that level as I have just realized now.
According to the Wiki, the seeds should have come out of my current crop. Why didn't that happen? (I do have a seed barrel in the stockpile and the farming activated)
Anything that blocks the path will 'disconnect' the two areas - if a set of up/down stairs have been changed to basic down stairs, you'll need to construct an up or up-down (preferable) staircase underneath your disconnected down stairs. It could also be something along the lines of a drawbridge being raised, or a door marked as forbidden, or a cave-in, or unexpected deep fluids - anything that removes the path between levels will cause your dwarves to no longer path between levels.
The seeds issue... if you've set the plots up to have a crop in the summer, you may not yet have any seeds - once you've grown a crop, it will need to be processed in some fashion - the seeds will be produced alongside the product. Any plant can be brewed, pig tails and quarry bushes can be processed at a Farmer's Workshop, etc. However, don't cook plants if you want the seeds - cooking destroys the seeds that would otherwise be produced.