Apparently I accidentally pushed some keys I wasn't supposed to, and the layer I was working on now bears that little padlock icon like the background layer does. And it made a cutout around the area I had painted, so that I can't paint outside that area. How do I fix this?

if the image is in RGB mode, double click the locked layer - it should open.

if it is in index mode, go to image>mode>RGB and then double click the locked layer.

I'm in RGB mode, but when I double click the locked layer, a window displaying the layer style properties shows up. And it doesn't look like it lists about about locking layers...or unlocking them. Unless I'm overlooking it? I do that a lot...

In the Layer Palette theres should be some icons at the top next to the word "Lock". With your locked layer selected, click on the padlock icon and see if it opens.

Hmm...that just makes the padlock icon turn black, and I can't do anything to the layout at all. Thanks anyway though. It's the same icon that the background layer has, and I can still draw on it, but only on the parts that I coloured previously. This is such a mystery to me!

Ohhh, okay. I figured it out - I feel so stupid now! I had somehow activated the "Lock all transparent pixels" option. I didn't notice but in the icon panel those are four different kinds of locks, not just the padlock. There's still so much more I don't know about this software...