Received a CP 2000 Notice from the IRS.  Do not panic.  But, by all means respond!
Each year, the IRS computer compares the information reported  from third parties on forms W-2, 1098, 1099, etc., with income and deductions reported on your individual  income tax return. This is called the Matching Program. If you by chance forgot to report any income, payments,  credits, or overstated certain deductions on a income tax return, the IRS may send to you a Internal Revenue Service Notice CP-2000. There will be an IRS telephone number that you can call to ask about the proposed changes that appear on the notice. About 20% of these notices are incorrect, so check yours closely.
The CP notice will inform you of the proposed changes to income, payments, credits or deductions, and the amount due to the IRS, or refund due to you. The first thing you may want to do is to pull out your tax return and verify your tax return to the CP Notice and the information that the IRS proposes to change. If someone has prepared your tax return, send them this notice immediately. Let the preparer do the work for you.
The first page of the CP-2000 is called the Summary Page. It provides a brief summary of the notice and instructions on what you should do to determine if you agree or disagree with the proposed changes. Remember, these are only proposed changes at this time. That’s it, proposed. You will have a time period to respond.
The IRS will point out  two things on the CP 2000. The amounts on your original return and the proposed tax changes. The notice will also show the decrease or increase in income. You will also see the name of the payer, their identification number, and the kind of document that was issued that is a  W-2 or possibly a 1099. It will also show the Social Security number of the person the document was issued to. The IRS tax liability is computed based on the income changes they find on the 1040 tax return. It can propose additional tax you owe the IRS, or it may show a refund due to you. If you feel the information from the payer is incorrect you should contact them immediately so they can correct their tax record and give the changes to the IRS.
The Internal Revenue Service also put a response page that is included on the form so you can  indicate whether you agree or disagree with the proposed changes. You may also authorize any party to represent you in your stead.
Should you agree with the IRS changes, you should sign the response and return it to the IRS. You can pay  the amount you owe within 30 days from the date of notice or you may send the signed consent without payment. The IRS  will  send you a bill for the amount due plus additional penalty and/or interest charges. You may request a payment arrangement to pay the amount you owe the IRS. If you wish to pay in installments, please complete and return the installment request form enclosed in the notice and return it with your response page. You will be contacted later with payment information. If an installment agreement is approved, you will be charged a fee. There are different types of plans the IRS offers, so chose the one that best fits your current financial state of affairs. The IRS Streamline agreement offers you a 5 year period to pay the tax back to the IRS.
If  for some reason you do not agree with part or any of the notice, go to option 2 or 3  on the response page, explain why you do not agree in a signed statement, attach the statement and supporting documentation for consideration  to the response page and send it back to the Internal Revenue Service. In any IRS correspondence include your day time phone number or cell number with area code and the best time of day to call.
You must respond  to the Internal Revenue Service within 30 days of the date of the notice or 60 days if you live outside the United States. Do not miss this time period. If you do not respond on time, the IRS will start sending out  notices and follow up with notices of levy.
If the Internal Revenue Service does not hear from you within the 30 or 60 day period, a statutory notice of deficiency will be issued and additional interest will be charged.
The IRS will start the billing process. This problem will not go away.  It must be addressed and fixed. The IRS sends out 1.4 million of these notices every tax year.