Accidentally? You can't accidentally confirm yes when the update message comes up.
Anyway you are screwed as you just installed System Menu 3.4 to a stub IOS50. You foolishly downgraded from 4.0 to 3.2 without installing IOS50v4889 which was the start of the problem for you.
Unless you have bootmii installed as boot2 then you have zero opportunity to recover this Wii beyond sending it back to Nintendo or swapping the motherboard.
Sorry I can't be anymore help than that.