First of all, do you have a paid account with Giganews and not just a 2-day trial which has run out?
If you DO have a paid account, is it unlimited, or have you used all your credits?
If it IS unlimited, then you should be fine. Of course you had to chance the server info to include your username and password, otherwise you'd not be able to get anything from Giganews.
Assuming something has went wrong, I'd simple Delete server and add it again, ie. entering server name (can be anything), address (has to be correct, usually news.giganews.com), port number, username and password. Make sure Server requires authentication is checked and you shouldn't need to check anything else, unless you use SSL. Click Add as primary and you should be set.