Without process auditing or accounting configured on your system I'm afraid finding out what happened will be near impossible.
Check out;
http://www.cyberciti.biz/tips/linux-audit-files-to-see-who-made-changes-to-a-file.htmland also the link lower on the page for process accounting;
http://www.cyberciti.biz/tips/howto-log-user-activity-using-process-accounting.htmlBefore you do this you should have a backup policy in place, auditing is fine but it wont stop it from happening again.