Can the IRS amend your return by themselves?

You have not provided much information for us.  You do not say what document(s) the IRS asked you to send or why you did not send it--or how long ago the IRS sent you a notice about it.  If you did not send the information the IRS requested then they can change your refund accordingly -- and you may not like the change they made.  TurboTax does not receive any information from the IRS after you file your tax return, so no one at TT know what the IRS is doing with your return or your refund.  Only the IRS can tell you.

To call the IRS:IRS:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

Listen to each menu before making the selection.

First choose your language. Press 1 for English.

Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.

Instead, press 2 for "personal income tax".

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

Then press 3 "for all other questions."

Then press 2 "for all other questions."  It should then transfer you to an agent.

 

The IRS will send a letter of proposed changes and if you don't disagree then the changes become fact.