Usually if you've been online on Gmail for an extended period of time and your browser's cache has overflowed.
Either clearing the browser cache or logging out and logging back in to Gmail will clear that condition.
It's just a Gmail server message.
[i]It happens to me every 2 or 3 days.[/i]
This is the entire message you've been seeing:
[b]'Your Network administrator has disabled chat in Gmail,' your network administrator or ISP has chosen to block chat in Gmail. If you've verified that your network has not blocked chat, as a first step, please ensure you have a good Internet connection and refresh your browser. If you're still experiencing difficulties, this may be caused by a conflict with software on your computer, browser cache that needs to be cleared, or a temporary problem on the Gmail server.[/b]
Here is the Gmail explanation:
http://mail.google.com/support/bin/answer.py?answer=34331
This message CAN be misleading but that's nothing new for computer error messages. ;)
:)