The IRS uses Notice CP31 to inform you that your refund was returned to them. Call the IRS at the phone number listed in the top right-hand corner of the notice & (after waiting on hold for an hour!) ask them to correct your address records & for instructions to get your refund.
Respond to Notice CP31
The IRS website has directions as well. They may ask you to file a Form 3911 to get a replacement check. They may send Notice CP32 to tell you they have sent another check.
Next, mail Form 8822 by certified mail to the IRS to change your address.
Contact Mr. Hopkins in Colorado Springs, Pueblo or Denver with questions or problems.