If you are unable to access your server via http:1.2.3.4:2222, then 1 of 3 things is likely happening:
1. DirectAdmin might not be running or
2. You have a firewall blocking port 2222.
Number 2 is easy to check by simply running (only on redhat systems):
/sbin/service iptables stop /sbin/chkconfig iptables off
Then test directadmin again.
If that didn't fix it, then you'd need to check your /var/log/directadmin/error.log to check for any errors as to why it isn't starting:
tail /var/log/directadmin/error.log
Common problems are: 3. Incorrect ethernet_dev set in the /usr/local/directadmin/conf/directadmin.conf file.
4. Invalid license, either due to wrong uid/lid, IP, or date. Try: Updating your DirectAdmin License manually
5. Your IP is blacklisted in /usr/local/directadmin/data/admin/ip_blacklist. This is controlled by the “brute force login detection” in Admin Settings. Use a setting no lower than 10, since even loading the login page counts as 1 failed attempt.
6. Binaries for a different operating system.
You can always try running DirectAdmin by hand (if it's not already running) to see what the problem is.
cd /usr/local/directadmin ./directadmin b200
to start it in the terminal with debug level 200. Use Ctrl-C to stop.