Cannot color under line art layer - please help!

[Zoodle]

Hi everyone,

I'm trying to color in some line art, but there's an issue with the line art layer. Whenever I make a new layer, move it under the line art layer, and try to color, nothing appears. However, I can see that there is color on the new layer, because it appears on the layer's "thumbnail" to the left of the canvas.
I also tried just going to the line art layer and using the bucket tool to fill it in, but it made the entire layer that color and even colored over the line art as if it wasn't there. I'm not sure what to do 

Has anyone dealt with this issue before? I don't know how this happened, and I've worked so hard on the line art that I really don't want to start over.

Thanks for reading!