Can't perform regular sudo commands, have to use su now

I'm an Ubuntu newbie I just installed 10.10 and I don't know what I did but now to perform actions that required sudo privileges (open synaptic, mount device, run apt-get install, etc) the OS won't let me saying I don't have enough privileges, for example when I try to sudo apt-get install it tells me:

myusername is not in the sudoers file.  This incident will be reported.

So I have to log in as su to do any on these things. I didn't have to do this before, I just used the regular root privileges but as I said I must have changed something unintentionally.

Can anyone help me?

The standard setup requires all sudoers to be in the admin group. You either dropped out of that group or are using an account that never had this group membership.

As root, add yourself to the admin group

addgroup myusername admin

And try again. If this didn't do the trick then something in your sudo setup is not as it should be. Maybe let us have a look at /etc/sudoers. I would expect there to be the last line:

%admin ALL=(ALL) ALL