On second thoughts, from the way I understand what you wrote. You failed to first create a new partition for Windows 10 to be installed on so it overwrote your Windows 7 completely. If you told the installer to clean the drive first and then install your stuff will be wiped. your missing files and folders should be in a folder created on your hard drive labelled Windows Old, unless you wiped the disk, then they are lost.
There are companies out there that can retrieve stuff from a hard drive after wiping,, but they're very expensive.
You didn't mention whether or now Windows 10 installation media is the full OS or a repair disk. If the latter then that's wrong media. You do the
free upgrade offered by Microsoft using their method.
If you wanted 2 OS's, 7 and 10 then you would have had to make a new partition for it using Windows 7 Disk Management tool.
Then installed on that separate partition.
As long as you haven't deleted anything you can
reverse the upgrade.
Your missing files and folders should be in a folder created on your hard drive labelled Windows Old, unless you wiped the disk, then they are lost.