Before I went traveling, I initialized an old Mac Mini i5 that I decided to sell and updated a new MacBook M3 to macOS Sequoia 15.6.1. When I was back and turned on the MacBook, which had been turned off, all of the files were gone. Nothing is on iCloud either. How am I supposed to get back the files?
So, the files are all there right after you upgrade it? It’s weird that the files disappear after you turn the Mac back on.
I think there’s still hope to get them back from Recovery Mode. I don’t recommend you keep using your MacBook because you’re risking the deleted files being overwritten by new files.
What you should do is to stop saving any files to your Mac and follow the guide below to boot into Recovery Mode and run the data recovery software to retrieve your lost files.