I have this same problem when it's booting from a cold start. There's no beep indicating a POST, and the monitor just shows me "No Signal Found". This happen after changing my power supply, but I learn to live with it though, since a restart usually fix this. This only happens when I first start the computer in the morning. I guess the system needs to warm up first. I haven't gotten it fix, it's probably as simple as replacing my PSU or adjusting some voltage level.
Have you made any changes to your system? You should check the PSU and the video card.