people like to talk about how stupid nintendo is and how buggy the IOS and update stuff is, but there is actually a good bit of testing and logic in place. for 1, nothing gets "halfway installed". the update process starts by installing all the IOSes. if it starts installing a IOS and then gets interrupted, that halfway installed stuff is just deleted and the existing version is still intact.
if you were doing the official update and it only part way completed, i say you have 2 likely possibilities for a brick.
1- you got the infamous 4.2 brick. just call nintendo and you will get a new wii with a 12 month warranty as soon as you mention this. there is no way you can fix it your self without a infectious and some know-how.
2- the update starts by installing all the IOSes. this includes the stub IOSes. so it replaced your system menu IOS with a stub. but it was stopped before it could install the new system menu. autoboot discs will not help at all here. since the very first IOS that starts on your wii is a stub. there is no way to boot a game. bootmii should be gone from boot2 if the update process made it to install the boot2 wad. call nintendo up and just say failed update. they cannot start the wii any more than you can without a IOS. same as above- new wii, 12 month warranty, only fixable with infectious and the proper knowledge.