You can use both your username and email for login.
Make sure you have entered right password. It is case sensitive. (password is different than PASSWORD)
Check your keyboard that CAPS LOCK is switched off.
Are you using Internet Explorer?
You may be having a 'sessions issue'. Close down your browser and open a new one. This sometimes resolves it.
Check your 'Privacy' and 'Security' settings. They are found in: Tools - Internet Options. If your settings are set above medium it will effect logging in. Reduce the setting to 'Low'.
Clear 'cache and cookies' - found in the Tools - Internet options