I have 2 EC2 instances (one window and one linux) in same subnet. I am trying to access one instance from another via private IP but I am not able to do so.
In security groups, both instances belong to same security groups and security group allows all traffic from the same security group.
I am not even able to ping one instance from another using private IP address.
Any pointers what could be wrong here.
This will likely be the result of Windows firewall being enabled, with default settings it will block ping access (among other things).
Disabling it should enable inbound/outbound access to work for ping. This is a temporary measure and should not be considered a permanent workaround.
You should enable whitelisting in the Windows firewall to allow the ports you would like inbound or outbound to have network access to/from other resources within the network.
More information is available in this link.