First of all be sure to be on the right webpage, "Log in" page.
Then, check you have correctly filled in the field with same email address you had used to sign in the first time and the password chosen. If you don't remember the password, click on "Forgotten password?" button in that page. Check you inbox: you will receive an email with all the information to set a new password.
If you signed up using Facebook, click on the "Login with Facebook" button. If a Facebook box pups on, you need to enter your email address and the password you used to sign in with Facebook.
If the problem lasts write us by filling the form that you can find in the Contacts page.