There could be several reasons.
1) Most of the time is that you have entered the email address or password incorrectly.
2) If you signed in with Facebook the first time, you will need to click the "Sign in with Facebook" button to log in.
3) If you still can't log in, you could refer to the following steps to reset the password.
Step 1: Click "Sign In" on the top right corner of our homepage.
Step 2: Select "Forgot your password".
Step 3: Enter your registered email address in the provided box.
Step 4: You may receive the reset email and you will create a new password with the link. Please also check your junk mail folder if you cannot locate our email.