sudo

What is the output of:

lsb_release -a; uname -a; groups

Thanks

Here are:
regis@regis-System-Product-Name:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.3 LTS
Release: 14.04
Codename: trusty
regis@regis-System-Product-Name:~$ uname -a
Linux regis-System-Product-Name 3.13.0-62-generic #102-Ubuntu SMP Tue Aug 11 14:29:36 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
regis@regis-System-Product-Name:~$ groups
regis bidon
regis@regis-System-Product-Name:~$

Then you aren't in the admin group so cannot use sudo.

Do you have other accounts that can use sudo? If so then get them to run:

sudo usermod -a -G admin regis

If you log off and on, then regis will be able to use sudo. Looks like you have been messing with group membership and botched it

Sorry but I Do not have other accounts that can use sudo.

Then use this guide:

http://www.psychocats.net/ubuntu/resetpassword

But instead of resetting the password, add your user back into the admin group using the command in #3

Hi,
I have read into details "How to reset your password in Ubuntu" and I have a good understanding.
But I do not understand: "add your user back into the admin group using the command in #3"
The procedure is:
passwd username
In my case what can I type ?

To re-add your user back to the admin group, its the command I gave on my 3rd reply....

You are just using the psychocats guide to get to the root console.....

Sorry for confusing.
I followed the procedure in recovery mode by using:
mount -o rw,remount /
usermod -a -G admin regis
The result is: the group admin does not exist.

Then run:

usermod -a -G sudo regis

Now it is OK
Thanks a lot.