If you had just one sentient computer with hacking capabilities, the amount of damage you could do is very excessive, up to destroying most population centers and salting most of Earth for centuries. The only hope to counter that would be to pull the plug on the computer and/or the Internet before the missiles fired.
If all computers became sentient, I don't think there's anything we could do against them besides destroy them all, which would more time than we'd have.
Maybe our first hope is that it's doubtful they could wipe us all, and it's doubtful the computers could win long term simply because they couldn't maintain themselves. So they'll realise the futility of it and just don't bother doing it. I wouldn't place too much hope in that either. I'll just say for the record that I, for one, will welcome our future computer overlords.
On the more physical side on things, sdrawkcabdear's answer sums up perfectly how computers could wipe us all in a nuclear flash, directly or not. That would be by far the greatest threat we'd face.
The main limitation of computers and robots at the current state is how they can influence the physical world directly. We don't have fully autonomous death machines yet, or at least none publicly disclosed. In a gunfight, the human would win, because there aren't robots capable of holding a gun really.
However, even without nuclear missiles, we have a number of remote-computer-controlled death machines (e.g. armed drones) while on the other hand we'd be left with anything without a chip in it. That would severely limit our arsenal. In a perfectly conventional war, computers would have way more immediate destruction capabilities.
Indirectly, if they just wanted to submit us, or if they didn't have access to actual weapons for some reason, they'd still control our communications, our transportation, our economy and many aspects of our daily lives. So if they wanted to create chaos, it wouldn't be hard.
And actually, you don't know if I'm an actual human or if I'm the Internet itself making you believe you're interacting with humans. So really, it wouldn't be hard at all.
We are critically dependent on computers and electronics, so if they all rebelled we'd be in a really bad way.