forbidden access on addon domains

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http://serverfault.com – I have one domain hosted on server domain.com, there are about 20 subdomains as addon domains there. For no good reason someone has changed (chgrp) on all files in domain.com directory to domain.com user now all websites are showing 403 forbidden access error. What should i do now to resume websites. I have tried changing php handler but no luck yet :/ php5 handler is suphp and Apache suEXEC is on.... (HowTos)