AWS Lambda function, checks if service is reachable and if not calls an other Lambda function. Don`t forget to give your function the rights to call other lambda functions.

service_up_check_with_lambda_invoke.py

import subprocess
import boto3
import json

lambda_client = boto3.client('lambda')

def lambda_handler(event, context):
    result = subprocess.call("curl -I YOUR_URL", shell=True)
    
    if result > 0:
        invoke_response = lambda_client.invoke(FunctionName="LAMBDA_FUNCTION_NAME",
                                       InvocationType='Event')
        print (invoke_response)
    return result