please mention what is your stack and how you are accessing the apis?
eg:
1)are you using ELB, api gateway, cloudfront, Route 53?
2)how is the client calling APIs? mobile app / Postman / Webapp / S3 website / third-party / external hosted site?
3)have you enabled CORS if you are using api gateway or ALB?
4)are you using on-demand or Spot instances
5)are you able to connect your instance from cli/console
6)how long this has been happening?
In general, basic thing you can do is it check cloudwatch logs for monitoring, which by default are updated once in 5 minutes.
If you have some budget, you can enable detailed monitoring for your instances
You can check with AWS Support if you have a Business Support plan