When this happens the formula component in the Smart Workpapers Add-in has disappeared from the Add-ins. One way of checking it is:
In Excel go to File>Options>Add-ins
Add-ins should contain both HowNow Smart Workpapers & HowNow Smart Workpapers Formulas for the workpapers to calculate correctly.
This problem is reported as occuring very infrequently. Here are two options for resolving this issue.
Logout of Smart Workpapers website, close excel & re-login to the Smart Workpapers website, open Excel.
Reinstall the Smart Workpapers Add-in from the home page of the Smart Workpapers website.