Number Typing Issues
Make sure you have 'NumLock' or 'NmLk' turned on. The worksheet will only respond to numpad/ten-key entries.
Make sure you've accepted all notifications that request enabling macros.
Ongoing Workbook Issues
If a workbook is acting up or not working properly, just close it and open it again. This resets all the parameters and you'll be good to go!
Worst Case Ongoing Workbook Issues
Something must have broken in your copy. No problem! Just download it again from here.