A little response for a BIG question:

Just revert the steps of your installation process.

For the BIG response, you must first tell us how did you install it !!! (the steps you followed when installing odoo)

Lines added after comment (from Duvan):

For the the second method from getopenerp all you have to do is to delete the odoo directory under /opt/odoo created at step 9, and if you want you can also delete the log and configuration files created at step  10  ​and 11.

I repeat, just revert the steps 9, 10 and 11.

For the first method from https://www.odoo.yenthevg.com/installing-odoo-11-enterprise-ubuntu/​  you must revert some steps inside the install script.

The most important part of this script is located from line 111 till the end of the script.

Begin by deleting the following files and directories (folders):

- enterprise folder and all its subfolders under $OE_HOME/

- custom folder and its subfolders under $OE_HOME/ 

- the server configuration file ${OE_CONFIG}.conf located under /etc/ ​

- the startup file start.sh located under $OE_HOME_EXT/​ 

- delete the odoo init file (initscript) $OE_CONFIG located under /etc/init.d/

- and at last delete odoo from the startup by applying the following command: update-rc.d -f $OE_CONFIG remove​

Hope this can help you !

N.B: 

$OE_HOME/ = odoo​

​​${OE_CONFIG}.conf = odoo.conf

$OE_HOME_EXT/ = /odoo/odoo-server

https://medium.com/@amirul_m/how-to-remove-odoo-completely-e62b1646b26c

Check out this link please 

I actually realized that before uninstalling odoo, the odoo server wasn't aware of my custom addon path so i just deleted the files in the home directory that has the name odoo.

After reinstalling it worked just fine.