1) Is this all chat forms? Have you tried both local and broadcast?
2) For an attempted fix, log into a toon that works correctly and, in the chat line, type: /chatsave
Log into the borked toon and type: /chatload
The chatsave command will save your chat settings to a default file. The chatload command will load the information from that file. Also, once you save a default file, it will be applied automatically to any new characters you create.
Paragon City Search And Rescue
The Mentor Project
Ironblade
Yes - it is all forms of chat. I can't even say "hi" locally. No broadcast. No team messages. No private messages. General announcements still show up in the chat box however. If I click on an emote the toon moves but no dialogue bubble appears. Happened immediately after using a terra volta respec. I've tried reloading the default file, logging off, verifying all files, etc. Nothing seems to work. It's actually amazing how limiting it is to suddenly be deaf and mute. 
I had not thought to try saving settings from another toon to overload on top of this toon's default file. Brilliant suggestion! I will try tonight. Thank you!!
Ironblade - you saved the day. Thank you. That worked perfectly! I owe you!
One of my toons - and it appears to only be one - is suddenly unable to chat. He cannot interact with other toons and no chat bubbles will appear and no text appears in the chat box. Anything I can do to reset the chat so it works again?
Thanks!