Gaming rig randomly shut off. Won"t reboot. Pretty new to the whole gaming picture. Please help.

[Biblicalwizardx]

Gaming rig shut off during game play from Sir, You're being hunted. Green LED on motherboard is still lit up. Power button won't work to turn anything on. How to i get it to reboot and get up and running. Here are the Specs by the way:
CPU: i5 3470
RAM: 8GB of Patriot Gamer 2 eries ddr3 (2x 4GB)
Mobo: Intel Dh77df mini itx
PSU: 500 watt sff
HArd Drive: 500 GB Western Digitial Blue.
Tanks for all your help.

[Muppet among Puppets]

Only that game?
Fans still spinning?
Case lights on on?

Green mobo light only goes off if psu is shut off

Make sure all plugs are firm