You have not told us enough. Was it working and then stopped?
Is it a new install that you have not yet had working?
More importantly, what video card are you using.
Do you have one monitor or two?
Do you have a KVM switch connected to your system?
Was the monitor that is giving that indication connected to the video card on boot?
When the video card sends out the wrong signal, some monitors say "no input signal" and some say "signal out of range".
Therefore, "no input signal does not necessarily actually mean that there is not signal.
It more likely means that the monitor can not handle the signal that is being sent.
After an update last night, my system with an nvidia fx 5200 is giving the same indication after I rebooted this afternoon.
Luck for me, my hp2035 monitors will print out what signal is being received when they power on. My system is now sending a 28hz video refresh to the monitor and it does not like it.
So, for now, I just rebooted that machine with the test-kde-2011.6 livecd and I will deal with the problem later.
If your monitor tells what the input looks like, check it and see.
An other thought is that if you booted the machine without the monitor turned on, the xserver will not be told the characteristics of the monitor and will make a wild guess which can cause the same indication.