This is a known issue of the older version of Microsoft Ribbon control. It will be fixed once we switch to XML Ribbon, meanwhile here's the workaround:

1. Hover your mouse over the Excel icon in your Windows toolbar. Two preview windows will appear: of Excel and the dialog box that you can't see.
2. Right-click on the preview of the dialog box and select Maximize from the subsequent context menu.

All set! At this point you should be able to see the dialog box.

Another common reason for losing an Enabler dialog box is when you had it displayed on the second monitor, but then disconnected your laptop from the second monitor. In this case the dialog will still have the starting coordinates on the second monitor and will not be visible. A very easy fix for that is to right-click on your task bar and select Cascade Windows. Here's more: https://www.howtogeek.com/310/bring-misplaced-off-screen-windows-back-to-your-desktop-keyboard-trick/