I accidentally removed a ppa and now I am unable to download any app from app center. How can resolve it? Also I don't remember the ppa, it was just the last one in the list.
If you added the PPA from the command line, you should be able to find (and re-run) the command with
history | grep add-apt-repository.
But there are no "default PPA settings". They are personal package archives; the default is no PPAs, so it's unlikely removing one would cause your symptom "unable to download any app from app center."
If you need to replace your default main source settings (e.g. /etc/apt/sources.list), this online tool can generate the files for you: http://repogen.simplylinux.ch/