Immediately mail in a form 8822 : https://www.irs.gov/pub/irs-pdf/f8822.pdf

Then call : 

Call the IRS:  1-800-829-1040  hours 7 AM - 7 PM local time Monday-Friday

When calling the IRS do NOT choose the first option re: "Refund", or it will send you to an automated phone line.  

So after first choosing your language, then do NOT choose Option 1 (refund info).  Choose option 2 for "personal income tax" instead.

Then press 1 for "form, tax history, or payment".   

Then press 3 "for all other questions."

Then press 2 "for all other questions."  

- When it asks you to enter your SSN or EIN to access your account information, don’t enter anything.  

- After it asks twice, you will get another menu.

Then press 2 for personal or individual tax questions.

It should then transfer you to an agent.