You will need to clear all of your cookies for the domain psychcentral.com.
Once you do this, you will be able to log into chat again.
In Firefox, this is under Options > Privacy & Security > Cookies & Data > Manage Data then do a search for psychcentral.com > Choose psychcentral.com and "Remove selected"
In Chrome, this is under Settings > Advanced > Privacy & Security > Content Settings > Cookies > See all cookies and site data > then do a search for psychcentral.com > Click on the trash icon next to any psychcentral.com entry
For other browsers, Google for "Delete specific cookies" and the name of your browser.
Once you remove all psychcentral.com cookies, it's likely you will need to log back into the forums. If you haven't logged out in a long time, you may not remember your password. So please make sure your email address is up-to-date in your profile in case you need to reset it.
Sorry for the inconvenience.