Commonly used excel files are suddenly missing

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Device
Mac mini, Macbook air
Operating System/Browser (if using the web)
OSX tahoe
Dropbox App Version (if using the app)
not sure

Question or Issue
two commonly-edited (except not the past 30d) excel files are suddenly missing.

They were not deleted and were present on multiple machines/devices as recently as early January. The only thing I changed was how I log in to Office 365 (changed from corporate license to university license). I haven't got it backed up in one place so am at a loss re how to recover them.