Could be lots of things, but the sinking pedal is not a good thing.

Sounds like the ABS unit might be leaking internally, and bleeding off pressure into an accumulator. I don't know why that would happen suddenly though.

Gotta check out the entire brake system, make sure nothing's leaking.

I assume you hit the brake pedal when you went to start it?

Usually, the brake light comes on either when the master cylinder reservoir is low, or the valve that balances the front and back brakes goes all the way over to one side when there is no pressure in one side. Not sure if the ABS unit finding something wrong will light the brake light, but I think so.

Let's work more on this...