"service atd start"
Most dameons (not all) have "d" at the end of the name so you can often guess the name that way.
service is a special command in RedHat variants like Fedora that will execute the init scripts in /etc/init.d.
You can use "chkconfig -list atd" to see what levels it starts in automatically - for example:
chkconfig --list atd
This shows my server only starts atd in run levels 3, 4 and 5. It doesn't run in run level 2 (the first multiuser run level) so if my system were at that run level it wouldn't start.
You can find out what runlevel you're in by typing "who -r" or "runlevel".
If chckconfig indicated atd should be running ("on") for your current runlevel it suggests that atd died or couldn't be started. You'd want to examine /var/log/messages to see if you could determine why it failed or died. Of course running the above "service atd start" may give you an error message if there is something preventing it from starting cleanly.