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I've never experianced this problem my self, but have you contacted your host to see if they've upgraded anything, php, mysql, etc.
Or if they perform back ups could they put a back up back on from when it was working.
Also try logging into the ftp for your site and clearing our the cahce and template_C I know this has caused some problems in the past with users logging in and posting with my own site.
Hope this is of help.