Actually, it was almost at 0% utilization. This is the first time we faced this issue. Instance type is c5.2xlarge. We have also checked the disk usage. It was at 55%. Is there a possibility of some internal OS error on Ubuntu, that might have crashed it and made it unreachable? If yes, where could I find the logs?
You should have a look at the Metrics for the instance. Especially the CPU Utilization, Status check failed metrics. See if there was high CPU utilization around the time you had to reboot instance. Depending on the instance type also look for CPU credit usage and balance.
Is it a recurring issue or one-off incident? What is the instance type?
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