Can you see the tabs on the bottom part? Try dragging one of them to the upper window. The might fix it.
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If you have more than one Tabs, right click on one and select "move to bottom".
Teams are the number one killer of soloists.
As mentioned, you can move a tab toward the top/bottom of the window or right click and press "move to top/bottom" to make it split again.
Personally, I'd just log onto a character that has it set up the way you like it and do a /wdwsave$$chatsave, then log onto the affected toon and do a /wdwload$$chatload.
I have a character, my main, that had the old chat UI with top and bottom on it. It worked fine for a long time (about six years) but recently the "bottom" part is gone. I still have the "top" part. But only one window. By losing the "bottom" I also lost the chat area. I tried \ window_resetall but no joy. Any help would be appreciated. I can't even find the option to add a "chat" tab to the existing window. There is literally a space where chat would be in my list of available options.