If my circuit doesn't work?

The most likely problem is that you have the current going around the circuit in the wrong direction.
The LEDs will only let the current travel in one direction. Reverse the + and - wires.

If your circuit still doesn't work when you've tried connecting it both ways round, then it's a fair bet that you've connected one or two of your LEDs the wrong way around. Check the leg lengths, look for the small cut-out on the edge of the LED lens or look carefully at the insides of the LEDs - they should all be oriented the same way.

Still not working? Check for short circuits and make sure all your joints are secure.

Finally check each LED individually (don't forget to use the resistor) to be sure you haven't damaged them.