gzip_compression() stopped working

I went to my blog (genealogyblog.com) tonight and found this message:
Warning: ob_start() [ref.outcontrol]: output handler 'ob_gzhandler' cannot be used twice in /home/hcadmin/public_html/wp-includes/functions.php on line 452
I couldn’t log in either. For now, I threw in a return; just after function gzip_compression() {, but don’t have a clue where to go from here.
I haven’t screwed around with the code for days. Does anybody have an idea what could have happened? (And yes, I haven’t touched, changed, added, or subtracted anything since Tuesday.)