If the callback fails for any reason you will need to send the user through the auth flow again. If the user got as far as connecting their org the first time, it will show as already connected the second time. They can continue to click through as normal to be redirected back to your app with a new code.
If you don't receive a response from a token refresh you can retry using your existing refresh token for up to 30 minutes. If you can’t refresh your access token in that time you’ll need to send the user through the authorization flow again to get a code that can be exchanged for a new access and refresh token.
Unused refresh tokens expire after 60 days. If you don’t refresh your access token within 60 days the user will need to reauthorise your app.
When you perform a token refresh, you should replace your existing refresh token with the new one returned in the response. If, for whatever reason, you don't receive the response you can retry refreshing your existing refresh token for a grace period of 30 minutes.