Monitor keeps going blank but is still runnin

Hello,
i need help because my monitor just keeps going blank after around 20 to 30 seconds sometimes longer or shorter, very random really but even though the screen is blank everything is still running in the background coz i can hear the sound still and even if i move the mouse or press a key it still doesnt come on however the green monitor light stays on all time so i have to turn it off and back on again for it to come on again and it just does the same over and over and over,so i have to keep turning it off and on to make it come back on all time and its annoying, please help ??? oh i think its a proview MA-782k 17" LCD monitor