Below is a link to an excellent, readable, short article on why it is important to answer all IRS notices by the date stated in the notice.
The takeaway if that waiting means you can lose your ability to challenge the amount the IRS claims you owe. Generally speaking, you have about 30 days to make a response and get it back to the IRS. Read that notice, gather your information, and contact the IRS. If you need help, please contact me.