do u have a nand dump from b4 the problem started? probably not since installing bootmii supposedly caused the problem
download my nus auto downloader, run it, type 4.2, then enter, then U, then enter, then follow the prompts until it finishes downloading a bunch of wads, then copy the wads to your sd card and install them all, and hopefully ur wii starts working
if that doesn't work, u could try comex's nand formatter, but research it heavily before using it, and make sure to install all the essential files, including EULA, RegionSelect, IOS36, IOS70, 4.2U System menu, etc.
btw, because your bootmii @ boot2 is still working, u should be able to use an emulated nand (ie. SNEEK), so worst case scenario, u can still access the system menu that way
edit: u dont have priiloader installed right? r u sure u didn't do anything else other than install bootmii, i still find it hard to believe installing bootmii caused these problems... I think its a lot more likely u r mistaken (no offense)