Today I meant to install software but mistakenly changed my administrator acount to standard. Now password not accepted, Please help me. i'm new to ubuntu (11.10).

I have edited your question to make it easier to read: I hope I have not changed the intended meaning. Try to keep questions in simple correct English it makes it easier for everybody to understand any not everyone here has English as their first language.

1. 11.10 is an old release have you considered doing a backup then installing 12.04LTS.

2. If for some reason you would prefer not to upgrade then open a terminal (CTRL+ALT+T) and give us the output of the following command

id

We should then be able to able to advise how to become an administrator again and reset your password

command is...
drwaqas@drwaqas-Inspiron-N4050:~$

authentification requires supreme administrator user pasword.but my previous pasword is not accepted

Press CTRL, ALT and T together this will open a terminal. In the window that opens enter the command

id

You should see something similar to this (no password required)

uid=1000(warren) gid=1000(warren) groups=1000(warren),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),107(lpadmin),115(sambashare)

We need to know what your system says

You will then need to boot to a recovery console and we can give you the commands you need to fix it

can you also run the command

cat /etc/group

Hi ,
try this
open a terminal (CTRL+ALT+T) and write

~$ passwd

it will ask for current password , leave it blank , just hit enter. If it accept this , then proceed and give twice a new password for your use.

IF above not works then follow this:

Alternatively you can use this method to retrieve your password and make your user admin again
http://www.psychocats.net/ubuntu/resetpassword

You have to give one more command except the commands reported in site. This

~$ usermod <yournickname> -aG sudo

to add your user to sudo group and become again administrator. Replace <yournickname> with your actual user name.

Thanks

Oh , and I agree with Warren Hill's opinion . Maybe is time to consider for a release upgrade to 12.04 LTS (supported until April 2017).

Thanks

Nick may have steered you slightly wrong in the last post I'm not sure but at some point the administrators group changed from admin to sudo. I think in 11.10 it was the old one but I am not 100% sure which is why I asked for the output of

cat /etc/group

if when you enter the command

usermod <yournickname> -aG sudo

you get an error that the group does not exist try

usermod <yournickname> -aG admin

instead. In either case replace <yournickname> with your user name

If you hold SHIFT at boot, select recovery mode, select root then run:

usermod -a -g sudo yourusername

then run:

reboot

You can now log in as your user and use your password to run admin actions.

yes i entered the word id.output is;
uid=1001(drwaqas) gid=1001(drwaqas) groups=1001(drwaqas),4(adm),20(dialout),24(cdrom),46(plugdev),111(lpadmin),122(sambashare)

Try the command I gave in root recovery mode

OK go into recovery mode as shown here

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

Then once you are at the command line (after selecting "root drop to root shell prompt"

Enter the following Command

usermod -aG sudo drwaqas

if you get an error that there is no such group then try

usermod -aG admin drwaqas

You can reset your password if you want then reboot. Job Done