Flash not working on Nikon D3100?

What's going on?

Added (1). When i put it on flash mode, the flash unit pops up, but when i press the button to take the picture, the flash doesn't go off. I can only see the flash in the dark. It is very very very dim.

Difficult to say. Either the flash is broken or it's fine but you have no clue what you're doing

Need more detail - In what way is it "not working"?

Doesn't pop up - have you got it in a mode that allows flash use? If yes then contact Nikon

Pops up but doesn't fire - may be faulty - contact Nikon