Maybe there hasn't been enough time for the letter to come in the mail. Do you have an IRS account? If so, I know that one of the tabs in the account is labeled Notices/Letters, but I don't know if all generated notices and letters appear under that tab or how long it takes to appear in that tab. But if you have the IRS account, it would be a quick place to look.
Another option:
Is there an IRS taxpayer assistance office in your area? They may be able to do it in person, and hopefully would also have access to any notices/letters. Here's how to find one:
https://www.irs.gov/help/contact-your-local-irs-office