If you cannot log in via mobile, it's possible there is an issue with your browser interacting with Facebook. When this happens, you will not be able to log in with your email, as your account is connected with Facebook.
To fix this, send us a request via our feedback form, stating the issue. We will disconnect your account from your Facebook, and you will be able to log in using your email.
To stop this issue from repeating, we recommend keeping multiple log in methods, such as by connecting your account to Facebook, and having a log in method via password. You can connect your account to Facebook or gmail by going to 'Manage Login Accounts' in Edit Profile, and connect the accounts you'd like to use.
To add your email, simply add your email and password under the 'Change Password' option. Click to expand your options.
To secure your account, verify it, which can be done under the 'Manage Login Email' options.