After the most recent update to IPhoto, which I implemented this month (May 2015), all of my "events" are gone. And I'm left with 17k photos completely unorganized. Is there any way to get those events back??
There are no events in Photos. Photos for Macs structures your Photos library automatically and creates Moments from photos taken at the same time at the same place. Related Moments can be viewed in the Collections View, and all photos taken in the same year in the Years view.
See this Help page on how to browse the Moments and Collections: https://help.apple.com/photos/mac/1.0/?lang=en#/pht56eafa987
Photos uses Moments, Collections, and Years views to automatically organize your photos and videos by time and location and make it easy to find just the photos you’re looking for. A moment is a group of photos and videos taken around the same time and in the same place—for example, an afternoon at the beach. A collection is a group of related moments—for example, your entire beach vacation. The Years view groups all of your photos taken in the same year together, so you can quickly view your entire library at a glance. You can also view your photos individually to see more detail and make edits.
As LarryHN pointed out, Photos created albums or each of your events on import, since there are no longer events. You will find them in the iPhoto Events folder. However, this are albums and not really events. To browse your albums and move items between the albums it is much more convenient to reveal the sidebar (from the View menu > Show Sidebar ⌥⌘S). Then you can simply add photos to albums by dragging them to the album in the sidebar.
See this support page: How Photos handles content and metadata from iPhoto and Aperture - Apple Support