The most frequent problem people have when logging in is not entering the username or password correctly. The password that was sent to you may have characters that look like other characters depending on your email font. (example: the number one and a lower case L look the same, zeros and O's look the same, etc.)
Also, everything is case sensitive so your caps lock must be off and the username and password need to be typed in exactly as shown in the email.
Best way is to copy and paste directly from the email into the log in form. That way there's no possibility of misreading the characters or mistyping. (remember not to grab any "spaces" before or after the text you are copying and pasting)