hmmm, CSS by itself should'nt cause a database error.
a number or things I can think off.
1. its not actually a CSS error, but a static message that is displayed all the time and hidden by CSS when its not in use, and you've overridden or deleted the class that hides it when not in use.
# this div will not display.
<div style="display: none;">
OMG REALLY BIG ERROR!!!
2. Its another error
3. you done something other than CSS. sometimes people put db connections (in php etc) at the top of the page, and if you where added css tags to the HTML page you might have changed this info by accident.