sure, you can get the icon back onto your quickbar.
Open the command menu. That would be in the very first tab on top of your window (attributes). Now, inside that command bar is one called attack
If you hold down shift and then left click it on the mouse, an icon comes up that you can drag back to your quickbar.
Also, F6 is probably already set at your attack command, so you don't have to waste a space for it on your command bar. It is recommended you either use F6 or perhaps assign another key (you can type /keyboard to open up the keys that are already bound to an action). You can make the letter "B" for instance your attack command, or even the space bar or something like that.