Undo/Redo Bar accidently deleted
I pulled them somehow out of the menu bar, so they became a window element. Then I wanted to shift them back into the menu bar, but clicked accidently on the "X". Now I can't get back my Undo/Redo Buttons. Unfortunately they are not listed in View-->Screen Elements. Disabling and enabling every single element in this menu doesn't work also. Restarting Hammer won't bring them back, either. Help me. 
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beecake's method will work,
you can also uninstall hammer, install it again and hope it works 
Hi,
Try this...
You need to change a registry setting.
First, close Hammer.
Open regedit.exe and navigate to HKEY_CURRENT_USERSoftwareValveHammerBarstate-Bar1
In there there is a setting called 'Visible', double click on it and change the value from 0 to 1.
Start hammer again and the menu should be back, floating and you can then put it back into the menu where you like...
HTHs,
Rob.
Wow, thanks. How did you find out that? Please don't say you just googled. That would be a fail from me.
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I deleted my undo and redo bar too when I first started mapping, it forced me to learn the keys.
Mapping Since May 21, 2011
I never knew there was an Undo/Redo button. I use the keys everywhere. When copying, inserting, undoing, redoing, writing. It is a very good shortcut 
Im glad you found out how