Are you plugging the monitor into a graphics card? If so plug the monitor into the onboard graphics and see if you receive a signal. Try a different cable. If you are using VGA use a DVI cable or visa versa.
The first question to you is this, bit of logic here.
Is it a new system ?you have just bought. And connecting it up for the first time ? Yes ? take the time to read all the instructions on it`s setup and where if it states to put the connections from the tower to a monitor.
Out of all the simplest thing`s it`s amazing people do this for example when there are often detailed instructions put there to help you, were people simply cast it to one side with no intention of reading it to cry and ask for help on a forum.
There is my bit of advice, it`s harsh but true in most cases.
Now to the latter of the point, you may of used this system for some time, but the fact you stated logic dictates with your statement its the first case reading the line posted in your post.
"when i hook up the connection cord with the monitor it says that there is not a signal" because if it was set up prior to it, then there would be no need to do this would there.
Now we have that out of the way, thinking logic here.
We need hardware specs to try to narrow the problems down, and trouble shoot your problem for you.
To help you, you help yourself, to help us to reach a resolve to your problem.
I've had the same problem, I've installed a new video + audio card and the problem is solved.
I don't know if it's intergrated vides & audio not good? I'm wandering if there is a way to check that.