Ping is not enabled to ec2 instances by default. A lot of guides tell you to simply allow all ICMP traffic through in the security group configuration. That is overkill. Simply add the following two rules to your security group and pinging the instance will work:

Custom ICMP rule -> Type: Echo Request
Custom ICMP rule -> Type: Echo Reply

While opening up additional ICMP types may be harmless, I always like to error on the side of only allowing what I explicitly want rather than allowing everything.