Experimentation is the key to learning new things in photography, and it should be something you do from time-to-time. Not only will it help you stay passionate, the more you experiment, the more opportunities you will discover. Long-exposure street photography is a great example of this kind of experimentation.
Go out shooting during the early hours of the morning or late afternoon on dusk when the light is low. Adjust your camera settings to increase or decrease the amount of light on your sensor. Take a mental note of what settings are favourable for the ambient light. Your shutter speed might be too long for the image you want to capture; as a result you may need to increase the sensitivity of the sensor by boosting the ISO, wait for a time when there is more light, or add light to the scene artificially.