Cpu turn on but no display

Its all started when im playing dota 2, in the middle of the game the screen suddenly became totally gray and there is no response no matter what i do(alt+tab, ctrl+alt+dlt). So i press the restart button, when it turns on the screen displayed some hazy green colored slash then there is no more display, the monitor displayed "no signal", usually this indicate that the cpu is turned off, but the cpu is on and there is many blinking light that show the cpu is on. Then i press the power button for 5sec then press it again. Now the display didnt even display any green slash, its just stay idle. The cpu still making the sound indicates it have been turned on like the "TIT" sound that came from the motherboard, but this time the motherboard is making another sound like "Teet", usually when i turn on the computer there is no this kind of sound. The keyboard still have the caps lock and numlock light and i can turn it on or off, there is no sound from the speakers
I almost forgot this happen after i just recentlyinstalled new driver for the vga and then try to play the dota 2. It lasted for 20-30min.

My computer spec :
Proc : i7-2600k
Vga : hd 6850
Memory : 8gb ddr3
Hardisk : 1tb
Os : win 7 home