I just accidentally deleted some files form /etc/ppp folder. After that internet not working I replace ppp folder from a live ubuntu but its still not working. I'm using ubuntu 16.04 LTS
Use dpkg to discover what files could be related to this file:
dpkg -S "/etc/ppp"
It will result:
resolvconf, pppconfig, pppoeconf, pptp-linux, ppp: /etc/ppp
Now, since you know the packages that uses this filepath. You have to reinstall them from the repository:
sudo apt-get install --reinstall resolvconf pptp-linux pppoeconf pppconfig ppp
Hope you fix your problem.