idle process status

[Michelle]

Could anyone please tell me what might cause a process to enter an I 
status or idle status.  I had two cron jobs which did not run a few 
nights ago, specifically the daily run output and aide.  The next day 
when I looked at the status of the jobs with the ps aux command, it 
stated that the status was I or idle for both of these cron jobs.  I 
was unable to kill the jobs even with kill -9 and had to reboot the 
server.  Now everything is fine and the cron jobs ran again last night; 
however, I would like to know what might cause this so I can prevent it 
from happening again.

Thank you,
Michelle