This may seem obvious but, please, make sure that you’re entering your Username (the email with which you registered in the service) and your Password correctly. Remember that the password is cAsE sensitive.
Browsers usually store usernames and passwords and try to fill them in, automatically, on websites. This sometimes replaces the credentials you explicitly entered. To ensure that you log in with the correct credentials:
1 - fill in the User Name
2 - fill in the password
3 - click on an empty zone of the site (this may avoid the browser to replace the data you’ve just entered)
4 - select the "Login" button
Best practice: regularly clean cookies and stored form information on your smartphone's browser. The browser may be storing outdated information, such as passwords that have been changed in the meantime.