No need to load a NAND backup. I don't think preloader can boot to a .elf file, and even if it could the one you used is the issue. Always load bannerbomb using one of the guides.
- Load Preloader (hold reset + press power), navigate down to Settings.
- Change autoboot = system menu
- and return to = system menu (or whatever you want to load when you exit a game)
Should be fixed up. Don't change autoboot back to Installed File unless you've loaded a .dol that's compatible with preloader. And if you aren't sure what that is, best not to mess with it unless you want to run into this again.
Make sure when working with Preloader to follow a guide, and not just try things at random. That goes for any softmodding until you're sure what you're doing.
You might consider updating to Priiloader as well, but it isn't required.
Also for the future, loading a NAND backup with Bootmii/IOS is possible but should anything interrupt the process you'll be left with a doorstop. A nice Wii-looking doorstop, but a doorstop.