Why can't I take a long exposure photo during the day?

I've done some long exposures at night with steel wool photography, but today I wanted to do a long exposure of a waterfall I saw. The camera (Nikon D3000) said the subject was too bright, and after taking the photo it was completely white, you couldn't see anything. I haven't changed anything in the settings but making the shutter speed a little longer (it was set for 15sec from my last project). How can I fix this?

It would be worth your while investing some time reading a decent photography guide book, you might find one in your local library…

You would need a 'neutral density' filter to help you achieve what you want… Here's a quick article with a few tips on the subject…'You Tube' might be worth searching too I reckon there should be a few tutorials on the subject.

http://digital-photography-school.com/8-tips-for-long-exposure-photography/

First, learn about the Exposure Triangle - ISO/Shutter Speed/Aperture.
http://digital-photography-school.com/learning-exposure-in-digital-photography

Learning the Exposure Triangle will help you understand why a Neutral Density filter is required for daytime long exposures.
http://www.cambridgeincolour.com/...ilters.htm

You need to decrease your ISO setting, enabling a slower shutter and a high f/stop. Personally I don't know why you want to do this. Under normal conditions a waterfall's movement require a very high shutter speed to stop the movement of the water. If you want the water to appear silky smooth then just set your ISO for about 125 and you f/stop high like f/22 and the shutter will slow down enough to blur the moving water.

Apart from all the other answers, you could also invest in an and filter.
It darkens the scene for you, thus enabling long exposures in daylight.

You're going to need neutral density filters to take long expsoures during daylight.

When you consider that a normal exposure in bright light will need settings of around f16, 1/250 sec at ISO 200, it's no wonder it turns out white (overexposed) if you lengthen the shutter speed! Your camera's light meter was telling you this.

Neutral density filters block light reaching the sensor and allow longer exposures. They come in different strengths, from very pale to almost black. These strengths are called 'stops'

Using the above settings, if you wanted to change the 'correct' exposure to a 15 sec exposure, then you would be changing it by 12 stops.

1/250 --> 1/125 --> 1/60 --> 1/30 --> 1/15 --> 1/8 --> 1/4 --> 1/2 --> 1" --> 2" --> 4" --> 8" --> 15" = 12 stops

Your best bet is to invest in a 10 stop filter and get a further two stops reduction by changing the ISO to 100 and stopping down the aperture to f22.

15 secs does seem quite a long time - you'll need to experiment to get the amount of blur you want - it depends on the ambient light you have and how fast the water is moving.

Alternatively, if you don't want to invest in a filter, shoot the scene when the light is much lower (at dusk for example).

Personally I don't know why you want to do this