A quick method, but don't use this if you are running any other programs that require constant internet access!
1. Disable your internet connection (see below for methods)
2. type /w a b [ENTER] into the chat box (this forces chat to look for the server)
3. Re-enable your internet connection
Wireless connection:
Click on the network icon near the system clock on the taskbar, and select Flight Mode to disable network. Click it again to re-enable.
Wired connection:
Type ncpa.cpl in a command box to open your network settings, then right-click on the connection to disable and enable it.
For those that cannot temporarily disable their internet connection, the widely reported work-round is to minimise the browser down to the taskbar for about 10 minutes after which chat will re-connect to the server and guild chat will work.