No signal display in monitor

Hello,
anyone help me to fix this problem? everytime i turn on my computer it goes ok the cpu turn on and it display black screen "no signal" before the windows start. even i restart my pc it goes well then suddenly before the
windows show up. this "no signal " display in my monitor.before the windows start.i checked the cable if there is loose one but its ok. my vga is new ATI 512mb. what seems to be the problem.well i can use my pc but this
no signal is annoying everytime i see it