Downgrading = dumb, I don't see a point to it anymore with safe updaters out there
Here's what likely happened. You downgraded from w/e system menu you had before to 3.3E and downloaded both the System Menu and its backbone (IOS30 v2576)
You also had IOS50 stubbed (which means mostly deleted) when you first upgraded.
Well, it found that IOS50 on your update disc was a lower version than the one you had (the stub's version number is higher than the real one)
So when you updated to 3.4E, you had the System Menu, but since the backbone of it was stubbed and useless, the System Menu can't boot up.
tl;dr you're screwed unless you have bootmii/preloader installed, you gotta call Nintendo and have them fix it