no internet access in AppStream 2.0 Stacks and EC2 machine also has ping status as "connection lost"

It sounds like you're experiencing internet connectivity issues with both your Amazon AppStream 2.0 Stack and your EC2 instance. Let's address these issues separately:

For the AppStream 2.0 Stack: By default, AppStream 2.0 image builders and fleet instances do not have internet access. To enable internet access for your AppStream 2.0 Stack, you need to configure it properly. Here are some steps you can take:

1. Ensure your Stack is using a VPC with a NAT gateway configured.
2. Launch your fleet in a private subnet within this VPC.
3. Update the fleet's network settings to use the VPC with the NAT gateway.
4. Make sure the security group associated with your fleet allows outbound internet traffic.

For the EC2 instance: The "connection lost" status for your EC2 instance suggests there might be network configuration issues. Here are some things to check:

1. Verify that your EC2 instance has a public IP address assigned if it's in a public subnet.
2. If it's in a private subnet, ensure you have a properly configured NAT gateway.
3. Check that your route tables are correctly set up to direct internet-bound traffic to the internet gateway (for public subnets) or NAT gateway (for private subnets).
4. Confirm that both inbound and outbound rules in your security group and network ACLs allow the necessary traffic.
5. Ensure your VPC has an internet gateway attached and properly configured.

If you've verified all these settings and are still experiencing issues, you may need to review your VPC configuration, subnet settings, and overall network architecture. It's also worth checking if there are any VPC endpoints or other network configurations that might be interfering with your internet connectivity.

If the problem persists after verifying these settings, you may need to contact AWS support for further assistance, as there could be account-specific or region-specific issues at play.
Sources
Internet Access - Amazon AppStream 2.0
Troubleshooting Image Builders - Amazon AppStream 2.0
Internet Connectivity Issue: EC2 Instance Cannot Access the Internet | AWS re:Post
Enable Internet Access for Your Fleet in Amazon AppStream 2.0 - Amazon AppStream 2.0

The resources are deployed in subnets in a VPC. Make sure the VPC has Internet Gateway and possibly Nat Gateway and you have correct routing rules in the route table. For a public subnet, the route table will look below. For private subnet, you can reference the nat gateway instead of IGW.

Also, note that, your security group must allow outbound connection to internet. For example, you can allow outbound port 443 to 0.0.0.0/0 to allow all outbound HTTPS traffic.