If your files are saved on the C: drive, then using OneKey to re-install will completely wipe all data on the C: drive, thus your files will be gone.
If you created a partition on the hard drive so that you now also have a D: drive and you saved your files there then your files will be safe. Onekey will only delete data on the C: Drive.
If your files are on the C: drive, then the only option you have is send in the hard drive to a company that does data recovery. This process is expensive, might be time consuming and is not guaranteed that all your data is recoverable. If even a small bit of a file is corrupted, the entire file is basically corrupted.