Monitor going off

I just purchased a new HP tower and hooked it up to my old monitor. The last time I used the monitor was 2 years ago and it worked just fine. So after turning themail computer on I turned the monitor on. Everything is powered up, green lights on, and the screen appears for about 2 seconds and then goes black. I tried repeating this on and off and it does the same thing. I checked all the connections so not what could I do??