There is one way back into an account here, and it needs the recovery key.
Enter it with your username and set a new password straight away. The key is retired as you use it and a fresh one is issued.
Then we cannot restore access to the account, and neither can anybody else — we store only a one-way hash of each key and cannot read it back. If you can still sign in on another device, create a key now from Settings → Recovery key. Otherwise write to our support team and tell them what happened.
Tap Share in Safari, then choose Add to Home Screen. The app opens full-screen with its own icon.