"Ich bin der Welt abhanden gekommen" by the Austrian composer Gustav Mahler (1860-1911), published in 1905 as part of his Ruckert Lieder after a poem by Friederich Ruckert (1788-1866).
http://www.recmusic.org/lieder/get_text.html?TextId=14001
You can hear a piano and soprano version
here:
http://ghim-misoon-ich-bin-der-welt-abhanden-ge-mp3-download.kohit.net/_/204937
I prefer the orchestra and baritone (voice) versions myself.
English translation:
I am lost to the world
with which I used to waste so much time,
It has heard nothing from me for so long
that it may very well believe that I am dead!
It is of no consequence to me
Whether it thinks me dead;
I cannot deny it,
for I really am dead to the world.
I am dead to the world's tumult,
And I rest in a quiet realm!
I live alone in my heaven,
In my love and in my song!