What to do when I accidentally kill the whole party?

So, my players’ PCs were fighting 11 fifth-level orcs. They are five 5th-level characters in D&D 4e. It ended in an unexpected TPK.

Thing is, their bard was new (he was put in place of their cleric), with not enough magic items to be as strong as the others and, as everybody knows, a bard is not as powerful as a cleric.

It was a damn hard combat, though my players were used to it and they used to play very well together, making some combats easy even when they were meant to be overwhelming. This one was supposed to be quite hard, but not overwhelming.

What happened was: the wizard miscalculated his Color Spray, hitting the fighter, who lost his Rain of Steel stance. They didn’t deal with most of the orcs during the crucial time they had with many dazed enemies, because of the Wizard’s mistake. It was brutal. They didn’t get a break after that for the whole combat and finally, were obliterated by the orcs.

Now I feel like I killed 5 characters, but only one had it coming. The other players were disappointed because they all feel like it was the wizard’s mistake that killed them all (and I also believe it, BTW). And the one who plays the wizard was the only one who was “Yay! Finally I get to play my hybrid wizard/warlord!” which pissed me off a little bit more, I’ve gotta say.

They are all making new characters but they feel like the campaign ended prematurely. They also don’t wanna go back to level 1, but for that matter, I don’t feel like GMing for brand new characters that already start as strong ones.

So, how to deal with this situation?