now it is impossible to tell what happened, but you can make arrangements so that next time if this happens you would know more.

you could modify original monitoring script that runs on your Antminer every 3 minutes:
/usr/bin/cgminer-monitor

cp /usr/bin/cgminer-monitor /usr/bin/cgminer-monitor_BACKUP

add new line before or after every:

    /etc/init.d/cgminer restart

the lines you want to add are like this:
    echo restarted at $(date '+%Y-%m-%d-%H-%M-%S') no process running  >> /usr/bin/avg.log
    echo restarted at $(date '+%Y-%m-%d-%H-%M-%S') because no valid work being done >> /usr/bin/avg.log
    echo restarted at $(date '+%Y-%m-%d-%H-%M-%S') because of miner count >> /usr/bin/avg.log


if the issue repeats itself, then you can check by command:
tail /usr/bin/avg.log
and at least see which of the cases had occurred.

if for example - no process running, then your cgminer crashed
if no work being done, then probably your or pools network problem.

you can have additional checks as well, by adding:
ping -c 4 POOLIP >> /usr/bin/avg.log
ping -c 4 RANDOMIP_THAT_IS_ALWAYS_REACHABLE_LIKE_GOOGLE.COM >> /usr/bin/avg.log
next to the echo commands and you will see if this was due to your or pools network connectivity - it will try to ping 4 times.