Ubuntu instance lost connection and crash

Hi, I am a beginner of AWS service, I've deployed some service on my instance, but I found that these service down sometimes and SSH can't connect to it. So I restart the instance and check the syslog. Here is what I found in log. "eth0: Could not set DHCPv4 address: Connection timed out" Can anyone help me to find out what reason caused this issue ? BTW, here is my instance ID if it's needed. i-05c242d2ca45d58ef