ubuntu root account accidently changed to standard account

This isn't possible. Root will always be root and can never lose power. If you mean your user account that you use sudo with then that isn't root, it's your account and only root is root.

Can you give the output of:

groups; lsb_release -a

Thanks

tanmoy adm cdrom dip plugdev lpadmin sambashare
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 12.04 LTS
Release: 12.04
Codename: precise

Yes, that's your user account, not root. If you hold SHIFT at boot and select recovery mode, then select root you can run:

usermod -a -G admin tanmoy; reboot

You will then be in the admin group and be able to use gksudo, sudo and kdesudo :)

when i put the the command its show 'admin group dose not exist'

What have you been doing!?

I suggest you make the group in the root recovery console, then add your user to it

please give me step by step instruction

http://linuxservertutorials.blogspot.co.uk/2008/11/ubuntu-create-group.html

group creation command give this error "cannot lock /etc/group; try again later". i also try adduser tanmoy adm, it show already in this group

Sounds like you need to fsck the partition in liveCD

i solve the problem with help of this link-http://psychocats.net/ubuntu/fixsudo