Word is crashed. After re-starting you get the a message similar to the following:
"Word experienced a serious problem with the ‘axespdfforword’ add-in. This add-in is incompatible with Data Execution Prevention mode and should be disabled or upgraded to the latest version. Do you wish to disable this add-in from loading?"
Read the message carefully!
If you click "No", then the ribbon tab "axesPDF" still appears in the ribbon. You can use the add-in as before.
If you click "Yes", then the ribbon tab "axesPDF" disappears because Word disables the add-in. This article guides you through the workflow to get your ribbon tab "axesPDF" back.