It's always very annoying when you can't log in anymore. In most cases this has to do with an incorrect combination of your email address and your password. Fortunately you can quickly restore your login details via this 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 these is therefore done via these environments.