Hardware My computer wont boot

I am using a Dell Opti Plex 780 desk top computer. It's proplems are that:
1. the light on the mother board is Amber and the fans are runing but does not boot
2.the green lights on the power button are flashing
3. On the monitor it says power saving mode
4. The key board does not show any sign of power but the mouse does
Those are the problems when I switch the computer on.
can you please help identify what is causing this.
Thank you