Gonna need a lot more info than that man...
Pls post your entire system specs.
Is this first time boot? Or it just randomly started?
What cables are you using for the monitor? HDMI, or DVI-D/DVI-I
Are you sure you plugged in the monitor cable to the GPU output and not the mobo if you have a GPU, if not, the other way around.
Where did you plug in your keyboard/mouse? Front panel USB, or MOBO I/O USBs? If front panel, make sure you connected your case's front panel connector to the USB 2.0/3.0 slot on the motherboard.
EDIT 1: In regards to the post above mine, yes please follow all those steps, and if the problem still persists, then come back with all the details, all you've tried, and what problems were eliminated via the troubleshooting.