Screen become black

hi friend,
i have a problem related to display.
during the use of torrent client i keep my pc on for around 6 hours.
from last few days i am facing a problem.i lost the display after 1 or 2 hours.
i mean everything is power on.
during that period monitor is in the power on state and motherboard is also working.
i just can not see anything on the scree what is going on.i do not know even during that time the downloading process happened or not.i have to restart the pc every time and then it works fine again.
now i do not even know that whether it is a problem of mobo or the monitor.
my system config
processor : phenom
mobo : m2a-vm hdmi
ram : 4 gb
psu : 600w
hdd : 2X1 tb
monitor :17" crt
_______________________________
Love Moves The World..........................