Have you tried to open a Microsoft Excel file in ArcMap and get the following error?

Error: Failed to connect to database. An underlying database error occurred. Class not registered.

I used to be able to open Excel files and now it stopped working.  What happened?  Well … our computers were recently upgraded to Microsoft Office 365 and when that happened it removed an important driver that ArcMap uses to open Excel files.

What to do?  Head on over to this ESRI Technical Support article and install the Microsoft Office system driver.  Don’t worry if you were using a version of Office other than 2007, the driver will work for you.  It fixed my problem!