It could be to not enough RAM available. If your service ran out of memory, it'll get killed and you can have problem like above.
You can look for logs. Start with syslog:
Look there for entries like OOM, OOM killer, proccess killed, out of memory. If you find that logs, it means that you don't have enough RAM.
If you can't find logs there, you can also look for alternative location
In case this is the problem upgrading Droplet or installing Swap would do the job.
How To Add Swap on CentOS 7 or for CentOS 6 How To Add Swap on CentOS 6.
Linux swaps allow a system to harness more memory than was originally physically available. Here's how to set up a linux swap file on CentOS 6