Bitmap layer almost invisible

I was experimenting with a little animation when the layer became barely visible.
I was on a bitmap layer with a custom brush that didn’t create lines that are too dark but are still visible but the all of a sudden the sketch became almost transparent.

I tried:
Control + z
Adding a brightness/ contrast layer to adjust it.
Using the repaint brush to try to darken the line.
Checking the layer opacity.
Changing the screen settings with the Intel Control Panel to increase the contrast temporarily.

Is there at least a way of selecting the alpha channel, like Photoshop, so I can then repaint it? My animation is unusable in its current state.