Here is the full list of all 21 Lost achievements.
Jigsaw puzzle completed
Click 19
Click 20
Click 1
Click 2
Click 3
Click 4
Click 5
Click 6
Click 7
Click 8
Click 9
Click 10
Click 11
Click 12
Click 13
Click 14
Click 15
Click 16
Click 17
Click 18