Forum Adding 'www' to URL shows you logged off
 

So, I have this problem, I don't know why it suddenly occurred if nothing was changed.
When members go to my site, http://example.com or just example.com, they'll be shown as logged on, completely fine.
but if they go via http://www.example.com or www.example.com, they'll be shown as logged off.
What's the issue and how can I solve it? Thanks!