[CloseOne]

The solution is to wait for the game to finish loading. Next, if the problem occurs you can:

1) Right-click the screen.

OR

2) Minimize game and type anything on a browser/notepad/chat/etc.

Once you do those and return to game, it should be fixed.

EDIT:
IF the error is that the keyboard stops working altogether (ie. Num Lock light does not turn on/off when pressed) do this:
1) Goto Control Panel
2) Goto System
3) Goto Hardware tab
4) Click Device Manager
5) Collapse Keyboard on the list
6) Right-click and uninstall
7) Return to Control Panel
8) Click Add Hardware
9) Follow the directions and find your keyboard on the list if a list does exist.
10) Install and you're done.