Possible reasons that the shutter button won't release?

I was trying to take a picure of a candle from a close distance with my kit lens(on nikon d3100). I was shooting at 55 mm with shutter speed 1/4, 1/20 1/60(i was playing with it, changing it). My aperture was F 5.6, iso on 1600, white balance auto, my focus mode was singe servo AF, my AF area mode was singe point AF, metering was on centre weighted. BUT my shutter won't release. When i pressed it donw nothing happent. My lens was on auto focus. And cause of slow shutter speed i have my camera on somewhere stationary.
This happened again.
Is it something on my settings?
What's wrong if anyone knows.
It happens often.

Added (1). Hi. The candle was not lighted. It was just for decoration. I had my camera on manual mode. Is it because it can't gained focus?

Depending on the mode the camera is in, it may not fire the shutter if the lens can't gain focus. This could be if your camera does not like focusing on flames (some don't) or if you are just too close to the flame (shorter than the minimum focus distance (note focus not focal).

Try setting the lens to MF!

If it's all dark just it will focus and release when direct to flame

Go into your custom menus. Look for "AF-S Priority Selection." The default setting is "Focus," which means that, in the AF-S mode, the shutter will only release if you have locked focus. You can change it to "Release" priority so that the shutter wil release regardless of focus. That's where I keep my cameras. You still get the focus confirmation indicator on the "half press," so you are not giving anything up 99% of the time.

You could also switch to the AF-C (continuous) mode where the default is already "Release" priority and you will get a picture even in focus is not confirmed.

Or, as others said, just use manual focus.

How close were you? Lenses have a minimum focusing distance. If the lens can't achieve focus (in AF mode), the camera will not take a picture. Try moving back a bit then shoot again.

As reference, my Pentax 18-55mm DAL kit lens can go as close as 25cm or around 10in.

http://keerok-photography.blogspot.com/2011/12/common-reasons-why-camera-wont-shoot.html

This is due to:-

If your subject is too dark and is not recognised by the camera, so it will have some problems in recognizing the candle.

In this case you can use manual focus.