I can't make my Nikon d40 shoot low light?

I have a Nikon D40-with a Nikon Dx 55-200mm lense. I'm shooting a wedding this weekend and I was just informed that it will be in a very low light setting. I have tried and tried to adjust my ISO on all settings to shoot these pics but when I adjust it-I can't make the shot-it just won't shoot. I push the button and nothing happens-what am I doing wrong? I'm a novice but am trying to learn-this is just a friends wedding, very informal and small but I want to do my best. ANY help would be appreciated!

Added (1). I also have a Nikon DX 18-55, would that be better?

Since this is a wedding I'm guessing you have a nice powerful flash since its a low light maybe indoors setting.
Just set up camera in manual, try to minimize the ISO since pictures may have a high amount of noise if set too high and may be useless when printed large. Now just test fire your flash to get a correct exposure or correct settings.
The lens you are using is not exactly the best lens for the situation but can work with the proper use of a flash.
If you do not have a flash or a "fast lens" I suggest you pick one or both up in the future so you are ready for anything that comes your way!

For most Nikons, you have to depress the ISO button, then turn the rear command dial for the ISO you want.

But you can also go into the shooting menu and set it there.

You will probably have to take your camera out of auto for this to work as when the camera is in auto - it sets the ISO automatically.

For heaven's sake, if you want to keep your friend a friend, practice with different settings and ISOs before the wedding to see which will provide an acceptable amount of noise.

On second thought, you might want to leave your camera in Auto mode. If you are not extremely familiar with setting the correct exposure in PSAM modes - changing ISO or not - you will most certainly botch the photo session.

Well it is a DSLR so yes it can shoot in low light. You would need a tripod though because less light means longer exposure times.

What troubles me is that you are going to shoot a marriage.while obviously not yet even familair with your camera. Seriously having a "pro" camera doesn't makes you a pro. A wedding is like a dance where you need to know when you need to be where and when and where and how you get those special types of pictures.

It is not something everybody with a DSLR can do. More I worry is that you haven't even scouted out the area, nor haven't planned what lenses you are going to hire. Nor do you seem to have a tripod. You as photographer need to plan with the wedding.know when what moments will happen and how you will get the best shot out of it.

Your two lenses are. Well. You can do weddings with them but seriously when doing weddings you need more then the two kitlenses. You need light strong lenses, a good 24-70mm for instance with F2, 8 or F4 through the full zoom range. It is a big heavy canon of a lens but it will let allot of light through aiding you in your search for low light photography.

I think in short you'll fail, not only because you lack the right equipment, you are not prepared [you haven't even scouted the area of the wedding] and probably don't know the schedule of "who what when where" I worry that at most your pictures will be the type that The Uncle can do.a wedding is supposed to be special it is a one time event. Getting pictures from there that The Uncle can do. Is not really what you expect.

You are in way over your head. Might I ask do you even have a second battery for your camera? A second or third memory card.

There's a reason the pro's cost a bit. That level of photography simply requires allot of stuff and learning.

Well even if you lack the experience and lack the understanding of the flow. At least rent some decent wedding lenses and have a tripod!

Possibly the lens won't autofocus in such low light, thus the camera won't shoot because not in focus. Try manual focussing. Otherwise use flash. That camera can shoot in low light without flash, but, like any camera, of course the shutter speed will be long thus you will have the problem of camera movement, also called camera shake, leading to blurred pics. Certainly only use the 'standard' 18 to 55 lens, since any movement is magnified by a long lens, and in general there would be no space to use a long lens. What gave you the idea that a long lens would be good?