Could somone please tell me what happened when I reboot my computer, it take too long to pass the part it said "starting sendmail"

I only changed the server name and when I reboot the server, all of the services are running except when it got to :

starting sendmail

it take toooo long to pass this stage.

Could someone please tell me what happened here ???

If your /etc/hosts file (or the "Hosts" section in netcfg) lists the loopback address (127.0.0.1, your computer's IP address when it is talking to itself) as having only a name that is an unqualified domain name - something like bla - you will experience the sendmail hang. If localhost and/or a fully-qualified domain name (FQDN), is listed, your system will not hang.

When sendmail starts, it attempts to find values for different macros - special variables internal to sendmail.

A better way, if you have a dynamic IP, is to use the dummy interface and assign it a non routed IP address (eg. 10.0.0.1) and route this in the kernel. For example, you could do:

ifconfig dummy 10.0.0.1
route add -host 10.0.0.1 dev dummy

Then add to /etc/hosts

10.0.0.1 myhost.mydomain.com myhost

Do reboot the system.