Taken from the NG FAQ:
Q: Once scouted, can I lose my ability to scout others?
A: Yes, if you scout people who don't take the Art Portal seriously, you may lose your ability to scout further people. You will still be able to submit your own art for as long as you remain scouted.
Q: Can I be unscouted?
A: If you break the rules or simply submit bad art - stick drawings, circles and squares, etc. - you will be unscouted. Your bad art will remain on your user page but art that breaks the rules will be removed.
I'm guessing you did one of these.
