I was working with my PC Running ubuntu based OS and then opened synaptic package manager to install something but the manager was having problem finding the source of packages , i opened the terminal
-> cd /etc/apt/sources.list then copied and changed the source servers , and replaced the original ones with this servers :

deb http://all.repository.backtrack-linux.org Index of revolution main microverse non-free testing
deb http://32.repository.backtrack-linux.org revolution main microverse non-free testing
deb http://source.repository.backtrack-linux.org BackTrack Linux Repositoryrevolution main microverse non-free testing

and again tried to install a program from synaptic , but this time it required GnuPG(PGP Public key).
Then again opened the terminal -> wget http://all.repository.backtrack-linux.org/backtrack.gpg && sudo apt-key add backtrack.gpg && sudo apt-get update

then installation worked well , i was able to install every package from synaptic , everything looked fine , until i rebooted the system.

After that every time that i'm trying to boot the old system it drops me to BusyBOX.
Please tell me for more detailed explanation .

This is the link of those files :
http://www.2shared.com/file/GQ3H0BW8/Sources.html

I'd guess you broke an important package somehow while using those non-standard repos.

Three things to try:

(1) Check for packages that have some state other than 'installed', and reinstall them.

(2) Check /var/log for hints:

(3) Reconfigure all of your packages