It's a good idea to never, ever downgrade your Wii. It's unsafe no matter what you do and unfortunately you probably bricked your Wii. There are, however different levels of bricking and some can be repaired. Did you install the WiiBoot (I think that's what it's called) or some other rebooting program?
Edit: Lol WiiBoot

BootMii is what I was looking for.