If the creator of the file added a "recovery record" (a special setting in WinRAR), you can restore it completely.
Before downloading a fix, it helps to know why this happened:
Avoid shaky Wi-Fi when downloading large files.
This guide will show you how to safely repair your files and get your data back. 1. Why Did Your RAR File Break?
Closing WinRAR or shutting down the PC while extracting.
Never unplug a drive while data is being written.
