Good shout on the built-in "Open and Repair" feature—that's saved me more times than I care to admit when a spreadsheet decides to randomly corrupt itself. Another lifesaver is checking if Windows created a shadow copy, though if the file's completely borked and you don't have a recent backup, you're usually hunting through temporary files in C:\Users\...\AppData\Local\Microsoft\Office\UnsavedFiles to see if a cache copy survived.
Honestly, dealing with corrupted client data and trying to trace back random file versions always gives me a headache, sometimes making me want to completely shut off my brain and just spin a random options wheel for what to have for dinner instead of troubleshooting spreadsheets all night. If the native Excel repair tools don't pull your data back out, third-party hex editors or specialized file repair utilities are usually your next best bet, though they can be hit-or-miss.