Microsoft Excel worksheets will not open

Hello I am running an iMac retina 4K, 21.5inch, 2017 with a 3.4Ghz Quad-Core Intel Core i5. I have 8GB of memory. I am running macOS Catalina, 10.15..

I have loaded Microsoft Office 2011 for Mac on my computer and have been using Excel and Word successfully for quite some time. Since I upgraded to Catalina I have noticed that existing worksheets of Excel and Word documents often will not open. If I restart the computer the problem is corrected.

Is this a known issue? Will upgrading to a more recent version of Office correct my issue?