Did you discover an expected letter from a message at the IRS "Where's My Refund" tool? Are you expecting a letter from the IRS?
Do you have an IRS account? If so, it has a tab for notices/letters. However, I'm not sure if all notices/letters show up in that tab, or how quickly it would appear after one is issued. But if you have such an IRS account, it would be a quick place to look.