When I get on my computer later, I'll see how you can fix it. Imo pressing ctrl z is way faster and easier.
You could usually add/remove things via View > Screen Elements, but I just turned all of those off, and that section is the only thing left. I'm not sure how to add that specific back thing though D:
Googled it and you need to do some fancy regedit stuff (not actually that hard).
First, make sure hammer is closed and open regedit.exe (just search for it in windows search).
Navigate to HKEY_CURRENT_USER\Software\Valve\Hammer\Barstate-Bar1\
There should be a value there that is labeled "Visible". Double click on this and change the value from 0 to 1.
After that, close regedit, reopen hammer, and the toolbar should be visible as a separate window. Just drag the window to where you want it, (you can also pin it back to the top toolbar by dragging it over the top of where you want it) and voila! You now have your buttons back.
First, make sure hammer is closed and open regedit.exe (just search for it in windows search).
Navigate to HKEY_CURRENT_USER\Software\Valve\Hammer\Barstate-Bar1\
There should be a value there that is labeled "Visible". Double click on this and change the value from 0 to 1.
After that, close regedit, reopen hammer, and the toolbar should be visible as a separate window. Just drag the window to where you want it, (you can also pin it back to the top toolbar by dragging it over the top of where you want it) and voila! You now have your buttons back.