Amazon App Says My Existing Account Does Not Exist

For the past two days I have been unable to log into my long‑standing Amazon account through the Amazon mobile app. Each time I try to sign in, the app claims that I do not have an account, even though I have been using this same account for several years without any issue. I have tried to reach Amazon customer service about this problem but have not been able to get through to anyone and therefore have received no support or explanation. I also attempted to capture a screenshot of the error message, but the app does not allow screenshots, so I cannot provide an image of the problem. I request that my account be located and restored, that I be allowed to log in again without errors, and that an explanation be provided as to why the app suddenly stopped recognizing my existing account.