IRS Notice CP27 informs you that you may be able to claim the Earned Income Credit to reduce your taxes or get a refund. This notice is very similar to Notice CP09. The IRS uses Notice CP08 to inform you that you may qualify for the Additional Child Tax Credit.
Respond to Notice CP27
First, review your Form 1040 as filed with the IRS. If you don't have it, you can order a Tax Return Transcript by calling the IRS at 1-800-829-1040 or use the website, or you can use Form 4506 to order it (for a fee).
Often, the IRS attaches Form 15112 to the Notice CP27 for you to fill out and return to them. You should probably do that. If any of the boxes in Step 1 of Form 15112 are marked as applicable to you, you are disqualified from receiving the Earned Income Credit. But if you receive nothing from the IRS within 6 months, you may consider filing a Claim for Refund. To actually make the claim for your refund you will have to complete a Form 1040X.