My Monitor is not working, please help?

we purchased a new pc today. I’ve made the mistake of taking the old one apart before taking the data off it. Ive set it back up again and turned it on. All was going well until the monitor showed the error message ‘no signal’ check your connections.
We have checked both the HDMI lead and the monitor with another PC, both are fine.
When you turn the PC on, it makes the ‘normal’ noises and no new/odd ones, the monitor shows windows xp loading then, where you would expect the login screen to appear it goes black and comes up with the error message.
Can anyone help?
thanks