My Computer turns on but does not boot.

My computer turns on but does not boot. There is also no beeping sounds coming from the speaker. I think it might be something to do with the fact that the power supply I am using with the Motherboard hasn’t got a four pin connector. However, the fan on the cpu spins, a little green light on the motherboard turns on and a light on the graphics card turns on. I also cannot see anything on the screen if I plug the monitor in.

Any help would be very appreciated.
Thanks in advance.