Tab Key Sticking

In case you find that your Tab key does not work please follow these steps.

The detailed story is below, but the solution to a “broken” tab key in a nutshell:

1. Press the “Windows” key (typically located between the left Ctrl and Alt) twice. This will turn off StickyKeys which can cause this problem. Stop here if this works. (Hat tip to Robert’s comment).
2. Press the “Alt” key twice.  Stop here if this works.
3. Press the “Ctrl” key twice.  Stop here if this works.
4. Uninstall the keyboard driver and reboot (details below).

find further troubleshooting in the below link. 

http://www.prescriptiveit.com/tab-key-doesnt-work-windows-office/