after I accidentally converted my external HDD (500Gb) to a boot drive to clean install Win10.
What a surprise all my other files are GONE

Admittedly, I am a total beginner with testdisk.
Is there a chance to restore some files? I want to restore as much as I can.
Do I need to format the HDD now that it is a boot drive with 32GB?
How do i have to proceed?
I'm very grateful for any help and advice