It's hard to know what the US would do for sure, they likely wouldn't publish the details of any contingency plans after all. However, as someone with some IT knowledge I can make some informed guesses about what will likely happen.
As implied by Brythan answer there are other resources for getting IT supplies other then China. Yes China is a large provider of resources, but they are not the only one. The US could start importing their supplies from other sources, including ones already located within the US. The supply of these resources would be lower due to the loss of Chinese manufacturing, which would result in raising prices as the lower supply struggles to meet demands; but it's unlikely that we would be at such a shortage that we would be unable to supply resources to critical systems. The US would simply pay more buy the infrastructure resources they need, the annual budget of the US is high enough to trivially handle even the worst case possible price increase caused by lowered supplies. It would be the every day consumers that would suffer more since they would be the ones that would end up feeling the price hike and buying less hardware due to the shortage.
Worse case would be that some infrastructure was dependent on a specific chip only made in China, but even then it's usually possible, if rather expensive, to replace hardware in those cases.
So in short it would be a little more expensive to support most of our infrastructure, as we work with lower supply causing price hikes, and there will be a few much large expenses to move away from any system that uses propitiatory hardware that only China could produce. However, our infrastructure as a whole will likely survive pretty well. Even the increased expenses would be trivial compared to the general war expenses.
To add a bit of further speculation I strongly suspect, though I don't actually know and can't prove it, that none of our really critical infrastructure uses Chinese manufactured hardware anyways. The US is fully aware of the fact that there are foreign agents that want to get access to our infrastructure for malicious reasons, with China being top on that list (Russia being #2). As such the US is likely concerned about manufacturers placing back doors into any hardware they produce and ship to the US for malicious, or espionage, reasons.
Thus I highly suspect all of our really critical hardware, our Nuclear reactors, government systems used by/supporting banking, everything at the NSA or White house, etc. is required to be either made in America or, at the very least, not made by one of the countries that are considered likely to try to hack us. This would be less for fear of the shortages in the case of a war as fear of back door hardware and hacking, but the end result is the same. The most critical systems likely do not depend on any Chinese infrastructure and thus will be no problem to support in the case of a war with China.