ec2 instance down twice today

I notice twice today that my instance i-0ebe95d0b2498f28b was in status "stopped". Each time I clicked "start instance" and everything seems to be working (the instance got new IP address as I'm not using elastic IP, and it seems to be running fine, at least for the first hour or so). This was a long running instance and for the past couple of months it was working all fine. So I wonder this is due to some thing wrong from AWS?