This happens rarely and can be solved by clearing cached content from Internet Explorer browser.
Please follow the steps below:
Step 1: Close CloudExtend Excel for NetSuite.
Step 2: Open Internet Explorer on your PC
Step 3: Clear browsing history and temporary internet files by following the instructions given here.
NOTE: Microsoft Add-Ins rely on Internet Explorer, not your default browser, for certain functions. It is important that you clear cache on Internet Explorer, not your default browser.