It depends on the context of the attack, particularly who and why.
A single nuclear strike would be rather bizarre given the capabilities of the nations that do largely possess a nuclear nuclear arsenal. Of course there's the usual suspect who might just actually be crazy enough to try it as a simple show of force against the US, at which point point I'd think it'd be over for that particular regime. China has shifted its official stance to a mutual protective alliance with North Korea, so if North Korea decide to actively be the aggressor - which any use of nuclear weapons blatantly would be - they're on their own against what is likely to be a very, very angry international community.
Anyone else though, against anyone else? That gets a lot murkier.
Any nation (other than you know who) that would be found seemingly culpable of launching a nuclear strike against another would probably be quick to try and deflect any sense of actual, national responsibility for it. They wouldn't know just how in the world one of their weapons managed to fire, and will launch an internal investigation immediately please don't retaliate and start a war. Except, in the likely event that thousands are dead, many nations - especially powerful ones with large militaries - would not stand idle. The US invaded two sovereign countries hoping to catch the people responsible for killing a few thousand people on one day; its not hard to see the same willingness if there was a death toll that exceeded that by quite some degree. Yet what if it was say, Sierra Leone that got hit? They would have to implore their allies to intercede on their behalf, and while there would be great shock and disdain at a nuclear assault, no doubt, they might be slower to react, especially if the suspects were another ally. Then you have countries that chiefly have regional ties, that may not be strictly allied to any nuclear power or large nation. They would have to plead for help from the international community in general, who might not be so willing to leap head first into conflict against a guilty nation that isn't as isolated as some others are.
That's even assuming the attacks clearly get pinned to one nation. If its terrorists setting off a nuclear weapon, ala Modern Warfare, who do you blame? Just the terrorist cell themselves? Sounds rational but no way would that satisfy the grieving. Their country of origin? Well, just because they're from a country doesn't mean that nation gave the ideology or drive to do such a thing. If you try to hunt down every connection, what happens if some of those do implicate members of a foreign government, but not the whole government?
Tl;dr, a bloody mess.
