HKEY_CURRENT_USER\Software\Microsoft\Shared Tools\Proofing
Tools\1.0\Override\en-US

I had 2 entries (DLL and LEX) pointing to non-existing files.  if you are not able to find these two entries then please follow the solution 2

Rename these to oldDLL and oldLEX.
 After that hopefully it will start working as normal

Solution 2

1. Click on Office button (top left) in word.
2. Click on word oprions button.

3. Click on Add-ins.
4. In the Manage box select “Disabled items” and click on Go.

5. In the dialog box select en-US spell checker if it is there and Click on Enable.