Sudo Failed

[Old Rocker]

When I try to do an update on my apt-get repositories, this is what I 
get:

$ sudo apt-get update
sudo: unable to lookup ubuntu via gethostbyname()

and when I click on Synaptic this is the error message that pops up:

Su returned with an error

Rebooting doesn't fix it, nor using other applications and seeing if its 
fixed, for instance the same message pops up when I use aptitude and 
not apt-get.

Something's happened along the way to change things (permissions or 
corruption) but I don't know exactly what.

Anyone know how to deal with this, and is reinstallation the only way?

-- 

Old Rocker