Brake Light Question

My on dash brake light (indication something is wrong with brakes) just came on today out of nowhere. I know it cant determine the condition of the pads, rotors, etc. so what does this usually indicate? And where should I start? Thanks.

Check the fluid level, and check the emergency brake cable. If the fluid level is low that could trigger the light. But another thing, if the fluid level is low, then that means you've got a leak somewhere. Check your lines all the way around. Odds are you've got a brake line thats rusted through, and is letting the fluid out instead of braking. If you didn't notice a difference in how the car brakes, then its probably one of the back brake lines. As for the emergency brake cable, sometimes if the e-brake is applied, and released it sticks and doesn't release all the way. This would also trigger the brake light.

I checked the fluid level and was empty, so there is a leak somewhere. If I fill it back up with flud, should the light go off?

It should go off if you have enough pressure in the lines a little bit after filling it, but if the leak is severe enough then there wont be a sufficient amount of pressure and the light will remain on until you fix the leak.