Shell Programming and Scripting

I'd like to create a new line using sed. For example, if a file contains one line:
abc

I want the result of sed to be two lines:
abc
def

Does anyone know how to do this? sed "s/abc/&/g" gets the first line, and I know there's some way to embed a carriage return after that. Have tried "\n" and "\\n" without success. Would be grateful for any suggestions.

BTW, this is a simplistic example. In reality I'm trying to modify one line out of an 80+ line file.

Thanks!

i believe other command does this job better. anyway, my spontaneous suggestion is:

sed -e '$ G' -e '$ s/$/new text line/' TheTextFile > NewTextFile

Thank you, Kanang. That seems to work fine. I also found a way to do this through perl:

perl -e 'while (<>) { s/(abc)/$1\ndef/g; print }'