From the tools menu, click Customize. In the pop-up customized dialog box, select the Keyboard button. The Customize Keyboard dialog appears. The button only displays if the keyboard has been customized. Select Reset All, and from the pop-up that appears, press Yes. The keyboard will be reset.
A change in any keyboard customization or assigned shortcut through reset disables styles and keystrokes assigned to macros. Consider checking and reviewing the assigned keystrokes before resetting them.
It is also wise to go through each keyboard assignment one by one and individually reassign them when in doubt over whether or not to reset them all back to OS default. If an important shortcut key is reset by mistake or a key needs to be reassigned, follow the same resetting procedure, but this time, at the Customize Keyboard dialog box, select Categories, then All Commands. Press New Short Key, Assign and Close.
If the reset is more than a simple shortcut key reset but a change in the language settings makes it impossible to type the correct letters, go to Control Panel, Clock Language and Region, Change Keyboards and Other Input Methods, Region and Language, Keyboard and Languages, and Change Keyboards to reset the keyboard.