To answer all the noobs questions about the end of Halo three i will explain what happened
The ark was busy building a replacement halo for the one you destroyed in Halo:CE.
The ring was only half-built when you activated it...via the index cortana had kept safe from the original halo.
Because the ring was only half-done when it fired off it began to shake itself to bits.
The ring fell into the ark causing a massive explosion.
The ark had sustained so much damage to portal leading to it collapsed just as the front-end of the Forward unto Dawn passed through it.
The ark was destroyed...and the halo ring didnt fire...the flood were destroyed in the explosion.
The back end of the Dawn didnt make it through the portal in time and chief was lost in space.
Play the level Halo (last level) of halo 3 on legendary and when you are done do not click a button and wait until the credits are gone it will show the memorial and then it will show you Cortana and the Chief in the broken half of the Dawn floating in space. I would also like to say that by the time that they are found it would be likely Cortana would be dead, so halo 4...probably not.
-Father of War
War is my name. Choas is my era. Death is my art. I AM AbuHarb
Abu=Father
Harb=Of War...AbuHarb=Father of War