Just got the answer in another forum:

Go to "My Forum Options" link just under the main banner above.
Clink on the "default CSS"  link,  copy/paste the resulting text from the 'default' area up into the "My CSS style" box.  Hit "Save changes"   All should return to normal.

(Although this doesn't answer "why it happened?'....)

RosemaryR
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"This...this is magnificent!"
"Oh, yeah. Ooooo. Aaaaah. That's how it starts.
Then, later, there's ...running. And....screaming."