there should be several tabs in your chat box; usually at least a combat tab and a chat/general tab. If one got closed you should be able to right click on the chat box and choose the option to open an existing chat tab. But if that doesn't work you can right click and create a new tab, then you gotta right click that tab and choose the chat options. It should bring up a menu with all the different chat channels; just pick all the chat channels you want for that tab.
This is all comming off the top of my head as I am at work, so it may be a little off, but the general idea should work....I hope....