If you get an error like the following when trying to open and load an Excel spreadsheet into your MXD, then you are missing a Microsoft Office driver. This can happen when you transition to Office 365.
The driver, named AccessDatabaseEngine.exe, may be downloaded from Microsoft at https://www.microsoft.com/en-us/download/details.aspx?id=23734 and I will also ask that it be added in the Software directory of the Members drive.
It just takes a moment to run and you don’t need to shut anything down. You might want to install it so you are ready to go the next time you need to work with a spreadsheet.