By default the Public interface has the public filter assigned to it, which should allow ICMP packets in and out. If you've played around with the filters or rules under Config - Policy Mgmt - Traffic Mgmt, you might have inadvertantly removed the ICMP rules from the Public filter.
Try removing the filter from the public interface and see if that allows you to ping. If so, then you need to add the ICMP In/Out rules to the Public filter.