Pc wont boot while restarting it...?

when i insert my card. ..system boot up nicely and works for about 6-7 hrs without showing any problem including gaming,,,,,,,but if i restart my system at the middle of my work the led screen goes black and shows no signal then after rest of about 2 to 3 minutes ..it again boot up nicely and works perfectly. ... the only problem occurs at restart my specs: core2duo e7500 4 gb ddr3 1333mhz kingston 500 gb sata wd 450 watt psu( recently bought) ga g41m combo gigabyte mobo 1gb gddr5 ati 6670hd