I have a problem with ec2 instance.

Hello.

What are you using for the instance type?
Also, is there any load on the CPU or memory?
When using t2.micro, etc., when the CPU usage rate increases, CPU burst credits are consumed to temporarily increase performance.
However, once the CPU credits are used up, the performance will drop to the baseline throughput, which will affect the operation of the application.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html

Have you checked the system logs and web server error logs for the time period when connection was not possible?

I assume that you using EC2 Instance Connect and SSH to your instance via browser. If you do nothing in the SSH console on browser, after a while you must reload the page to continue to using.