From my experience, the IRS does make a strong effort to notify taxpayers when issues exist with their tax account. Most of the IRS notices are standardized forms that can range from you having unreported income to having a refund on your tax return that is different than what your tax return states. The IRS is very good about using the mail to notify taxpayers, so do not be fooled if a person calls you on the phone and acts like they are from the IRS. This is likley a person just trying to impersonate the IRS to steal your money. However, if you receive such a call, remain polite but do not give out any personal information. Since the IRS uses mail and a primary method to reach taxpayers, it is important to keep your address with them current.
Your response to the IRS issue contained in the notice needs to be tailored to the issues involved. Often, I find that you have to clear and concise when responding to IRS notices to make any progress in resolving the issue. If the notice relates to an unfiled tax return, the IRS notice will ask for a copy of the tax return filed, or why it was not filed (this could be because you had to little income to cause a filing requirement). If you have many years of unfiled tax returns, its a good idea to hire a tax attorney to help file the tax returns and reduce the tax penalties.
If you ignore this request related to unfiled tax returns, the IRS will prepare a substitute tax return, using information that they receive from third parties (banks, employer W2 forms, etc.) to prepare the tax return for you. In some ways that may seem ideal, but this return the IRS will not allow a married filing jointly form of filing, or take into account any itemized deductions. Therefore, your tax bill will be higher than if you prepared and filed your own tax return.