UPDATE: Chrome changed how this work. Please see the fine answer from @JDGillis on how this works now (mid- and late-2020), not my answer below. Of course, it may change again in the future!
According to https://support.google.com/chrome_webstore/answer/2664769?hl=en:
To show extensions you've hidden, click the right side of your address bar and drag it to the left. To bring hidden extensions back to your toolbar, click More More. Right-click the extension's icons, and select Show in toolbar.
Some extensions don't have this option.
This is how it looks like when the cursor symbol changes.
This is NOT obvious but it works if you do it exactly: grab the right "edge" of the address bar (just to the right of the Add-Favorite star), and drag the edge to the left. Any hidden extensions show back up!
I'm not sure what the other suggestion from Google is for (right-click on the icon), as I didn't need to do that to keep the icon back visible again after dragging the address bar to the left.