Can you determine exactly when iMac was disconnected from power?
You would need to look through the system log for the last "Shutdown Cause" code.  

There won't be anything in cron because it was deprecated in favor of launchd.  However, launchd (or cron) wouldn't be of help because those logs show when things are started or error out, seldom do they show when things are shutdown - especially if powered off properly (no error)

To search the log, use the following command:

$ log show --predicate "process == kernel" | grep -i "shutdown cause"


See this answer for further details on how to search the log.