Hah, strange. Are other shelves empty too?
At the leftmost part of shelf panel there's a small square which, if clicked, will give you access to the ShelfEditor (same Windows>Settings/Prefs>ShelfEditor) - see if there the PolyModelling panel is really empty. If so, then the settings seem to be corrupted. If nothing fancy about your hotkeys, plugins and layout settings, then easiest would be to reset settings completely by exiting Maya, deleting entire C:\Users\User\Documents\maya\20XX\prefs folder, re-launching Maya - the default profile with all icons will be recreated. Otherwise, you can add your own (frequently used and customized) icons to the shelf by ctrl+shift+LMB-clicking any top menu item.
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Maya2019.1 @ Windows10 & GeForce GTX1080Ti
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