If you are D3-2 then you can't boot backups, no matter what they are for. It's also unlikely that you'd be able to boot any useful unbrick disk on 4.2 anyway, even with a modchip, unless you patched your IOS70.
Installing a bad Wad wouldn't normally cause Preloader to stop coming up, so the reset button thing should work. You might like to try a hard power off, leave it for some minutes and try again.
You can also try maintenance mode, start the Wii, at the Health and Safety screen, press and hold - and + and tap A button a few times, you may get into the Wii menu and have the ability to load Wad manager.
What was the bad wad you installed?