The error is caused by the fact that the port that you are using (if you did not specify anything, most probably it is 8069) is used by another process.  Using `netstat -plntu` command in terminal can show you which process is holding that port.  You might want to either select different port or kill the process that use the same port first.

Try the following :

1./ On the command line fire the following command ps ax|grep openerp
2./ It will list out the processes where it will find the word 'openerp'
3./ Look into that list whether any process of openerp is already running on the same port that you are trying to start the service
4./ It may also possible that any other process may be using the same port that you are trying to use for openerp server

You need to check whether the port that you are trying to access isn't used by some process

Try following commands to check the ports

change to superuser
1./ lsof -i : <<port number>> ---> this will list out the process that is using the specified port
    example : - lsof -i :8069
2./ netstat -tulpn ---> this will list out all the process with port and program

If you find that the port is in use by some other process or say openerp already than you will have to kill that process

sudo kill -9 <<process id>>

Hope this helps !!.

1.open terminal

2.type as,

~$ ps aux|grep odoo

3.it lists as follos,

odoo      1164  0.4  2.0  80648 40788 ?        Sl   10:22   0:00 python /opt/odoo/odoo-server/openerp-server -c /etc/odoo-server.conf
amz02     2473  2.5  2.0  79244 39100 ?        Sl   10:24   0:00 /usr/bin/python2.7 -u /home/amz02/Projects/odoo/odoo-8.0-647afdf/openerp-server
amz02     2647  0.0  0.0   4688   820 pts/0    S+   10:24   0:00 grep --color=auto odoo

4.find the number of odoo-server.conf(here it is 1164)

5.type as follows,

~$ sudo kill -9 1164

6.enter pasword

7.restart server..

Thank you all , it woks now :)