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Close Excel. Then hold down Option key on your keyboard, click on Go menu and select Library. This will take you to the library folder of your home folder. Open Preferences folder and trash the file 'com.microsoft.Excel.plist'. Then open Excel. That should bring it back.
If that didn't work, then close it. Hold down Option key on your keyboard, click on Go menu and select Library. This will take you to the library folder of your home folder. Open Preferences folder and this time, trash the Microsoft folder.
Go back, then open Application Support folder and trash Microsoft folder. If you don't see the folders there, then open Containers folder in library and trash Com.microsoft.excel
Try Excel. Please let me know if that didn't work, so we can continue troubleshooting.
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