You generally have 30 days to respond to an IRS notice, so there's no reason to ignore it. Always check which tax year the notice relates to. Do not assume that it relates to your most recent tax return.
Follow the instructions. It may request more information or ask a specific question. If it's a minor issue and you are confident of the facts, you can respond to the notice yourself. But sometimes, you may need to seek help from a tax professional. The IRS says in its article "Eight Things to Know if You Receive an IRS Notice" that "most correspondence can be handled without calling or visiting an IRS office."
If you filed the tax return with TurboTax, you can take advantage of our free audit support options. Our downloadable Audit Support Center walks you step-by-step through responding to an IRS notice.