problem downloading software packages

 i have installed UBUNTU 10.10 in my delL LAPTOP.

i am trying to install updates and software packages from the internet.But, its not working. it is giving me a error message that " update failed: check your internet connection".

my Internet is working properly. i am browsing, downloading and doing evrything else on the internet, but i dont know why it failing to install softwares and updates? it was working till yestersday

plz help