In 99% of cases this will solve your problem, so please make sure you have tried this before writing to support.
1. Make sure you are using the correct log in information
To get your password by email.
Click the “Forgot Password” link on the log in form.
Enter your email and click Send.
In a couple of minutes you’ll receive your password. If it doesn't arrive, check your bulk email folder.
2. Make sure you don’t make a mistake with the log in information
If you did a copy/paste from the email, make sure you didn't grab any extra symbols.
If you key the information in manually, make sure you don’t have the CAPS LOCK turned on and that your keyboard is in English layout
3. Make sure your browser is not the problem
If you received your password by email and are absolutely sure that you’re using the correct information, but it still doesn't allow you to log in, it could be a problem with your browser.
Clear the cookies and cache of your browser and try again.