Do not panic, lost files sometimes occur after you upgrade to Windows 10. You can look it up and restore your missing files.
When first you know about Windows 10, you may still hesitate to do the upgrade. The fears such as lost files make you reluctant to upgrade to Windows 10, but what bad things that might happen, Microsoft would not release a crazy Operating System that removes all user data right? Well shit happens when you finally decide to upgrade to Windows 10, all your files are vanish from the earth, you can not find them anywhere. You might miss it, but Microsoft has warned you to backup your files before run the upgrade, and now this what you got.