If the site software doesn't give you any errors, but says its logging you in, and then it doesn't log you in, please try the following please try deleting all browser history.

In Firefox, you can do this by clicking Tools -> Options -> Privacy -> Show Cookies -> Remove All Cookies
In IE, you can do this by Tools -> Internet Options -> Delete under Browsing History -> Delete All

Additionally, you can try restarting your PC.

When logging in, always make sure the "Remember Me?" box is checked.

If this doesn't fix your problems, make a new thread here.