Sometimes when you create a new layer it is initialized as "Not Visible".
I think if you create a new layer in the middle of an animation (not on frame 0) it is set to be invisible up to the point of creation.
The timeline for the layer turns "salmon" to indicate it is invisible.
Just go to frame 0 double click the layer in the layer palette and turn visibility on.
Then delete the key frames for visibility.
This happens to me all the time... the first time it happened I was like you... totally completely perplexed and confused.
-Vern