How do I find missing Excel File on iMac?

Provided you have the Spreadsheets category selection in System Settings > Spotlight, you can issue this command in either the menu extras Spotlight 🔍 tool, or in the search field of an open Finder Window. The '>' symbol stands for after so this will look for any Excel spreadsheet modified after Jan 16 of this year — provided it is saved to your local drive and not on some cloud service:

kind:excel modified:>01/16/2025

In excel you use the Open Recently feature, or you can use Finder, or a third party app like FindAnyFile or restore from your Time Machine backup. You have LOTS of methods available!!!