This issue is often referred to as the WordPress white screen of death. It usually happen because you exhausted the memory limit. It could be caused by a poorly coded plugin or theme. It could also be caused by unreliable web hosting.
It could also be that you are typing the correct username and password combination and you won’t be able to login. When you try to reset your password, you never receive the email. This can happen if you were a victim of a hack. We would recommend that you reset your WordPress password from phpMyAdmin.