It's always very annoying when you can't log into your LessonUp environment anymore. In most cases this has to do with an incorrect combination of email address and password. Fortunately you can quickly restore your login details by clicking on the following page: https://www.lessonup.com/en/forgot-password

Good to know: if you log in via Office365 or Google, you cannot change your password via LessonUp. These passwords are managed by Microsoft or Google. Changing and/or restoring them has to be done through their own environments.