For the original hardware, most "fixed" save data issues relate to physical corruption or connection errors.
If the data is truly unreadable but you have access to a modded Wii, you can attempt technical recovery: save data resident evil 4 gamecube fixed
or Action Replay can connect a GameCube memory card to a PC to download or backup save files. Dolphin Emulator : You can import your physical saves into the Dolphin Emulator For the original hardware, most "fixed" save data
To get a save file with everything unlocked (Chicago Typewriter, Infinite Rocket Launcher, etc.) onto your GameCube: Method A (Wii + GCMM): For the original hardware
If your save is truly gone and you don't want to restart from Chapter 1-1, you can download community-made saves that have everything unlocked.