I think I'm having the same problem as Kowze.
I followed the tutorial
here. Had the 3.4u firmware and downgraded to 3.2u from those steps. The 2nd to last step "+Load "Install Shop IOS51". Say Yes to installing IOS51 and Yes to patching a fix but No to the Wii Shop Channel." is what I think the issue is. My Wii froze after installing IOS51. I had to do a hard power off to get it to respond.
The Wii runs just fine. However, whenever I run the HBC, it gets stuck at a black loading screen. When this happens, my wiimote turns off and the Wii will only respond to a hard power off. I also have BootMii installed and going to the HBC through that also results in the same black screen. I don't believe my wiimote turns off though with BootMii.
I used bannerbomb to install the HBC - I've tried to reinstall it but it doesn't seem to do any good. I've also wiped the Wii entirely (using the Wipe/Format Memory option in the Wii Settings) but it still gives me the same black screen issue when I reinstall the HBC.
Does anyone have any ideas? What nabster92 said above seems to make sense. I think if I can remove IOS51, that may solve the issue and make the HBC work again. I would need to do it without relying on the HBC though. A friend suggested that I upgrade to 4.0 and do the downgrade again. I'm not sure if that would do much since the memory wipe/format didn't help.