Unable to paint, draw, erase on layer

I have been editing a batch of photos and everything had been working fine.

But then it was not allowing me to directly edit the layers. I cannot draw, paint, erase, smooth, etc. I can draw shapes and select and delete material, but have no independent control with the tools. Any ideas?

Locked layer?

Whats the color mode?

If a layer is locked you will be unable to edit it. That might explain why you can't draw on it.

To check your color mode as tornupinside suggestions go to Image > Mode