I accidentally clicked the "hide" link when I was trying to switch from main to sales chat, and now I cannot un-hide it. There is no link, just a blank space where the chat box used to be.

This happened to me too. I think in the game layout update the color of the link was changed to be the same as that background color. If you click/hover around near the top of where the chat box should be the link changes to a visible color.

I found it! Just like you said, the link is the same color as the box background. Was able to hover around with the cursor until it highlighted.

Fixed, thanks.