As an AWS Certified Developer Associate, you are tasked with managing an application running across multiple Amazon EC2 instances. These instances are fronted by an Internet-facing load balancer which handles incoming client requests from the internet and distributes them to the EC2 instances. Additionally, the load balancer has a health check configured to ping the index.html page located in the root directory of each instance to determine their health status. Recently, visitors accessing the website over the internet have been experiencing timeout errors.

What should be checked first to resolve the issue?