No picture on screen on startup

I have recently picked up a Compaq SG3-230UK that I'm trying to fix for a friend, but am not getting very far. The problem is when the computer is turned on there is no picture on the screen at all, the LED light on the monitor continues to flash, which would normally mean no PC input. The fans are going so it's getting power, but that is the only sign that it's alive. Even the HDD light doesn't flash at all. Does anyone know what might be happening, any help is much appreciated.

Is that actually a HP Compaq SG3-230UK Desktop PC?
What is the exact serial number(S/N and exact product number(P/N) on it?
Does it still have all of its original hardware?

Yes it is the desktop PC, the serial number is CZC044BRVP, and the product number is XH840EA#ABU
Yes it also has all of its original hardware, it's never been modified in any way.

Yes it is the desktop PC, the serial number is CZC044BRVP, and the product number is XH840EA#ABU
Yes it also has all of its original hardware, it's never been modified in any way.

As I feared might happen, the HP Compaq support site no longer lists that SG3-230UK because of its old age.

Did your friend tell you when the last time was that desktop worked okay?

I'm not a diagnostic expert, but my first guess would be its hard disk drive(HDD) has failed.

Even if the hard drive had failed you still should have video and be able to get into the BIOS by pressing F10.

Have you verified the monitor is working OK.

Compaq SG3-230UK download instruction manual pdf
Just a moment...

I'm not sure when the computer was last used, but I don't think it was too long ago. As far as I know it was working fine when it was last used so I don't know what could have gone wrong between then and now.
I can confirm the VGA cable/monitor is not at fault as I first looked at the computer at my friend's house, and after having no luck with it there I brought it home and connected it to my monitor. At first I also suspected possible hard drive failure, but realised if this was the case I should at least get the startup screen with options to get the Bios, and then possibly it telling me "error in Drive C" or something similar.
So I'm a bit puzzled to what might be happening, I'll have a look at the link in the previous post and see if that answers any questions.

- - - Updated - - -

Unfortunately the document hasn't provided any answers so if anyone has any other ideas I can try it will be much appreciated.

Does the computer have a graphics card? If it does you could try to reseat it.

I have unplugged the hard drive and turned it on as normal, unfortunately the same thing is happening. So my guess is the fault is with the graphics card, but the problem is that the computer only has an integrated graphics card so I'm not able to reseat it. Also there is no slot on the motherboard for me to insert a different graphics card. I thought you might be right in saying the hard drive is at fault as the hard drive light doesn't flicker at any point. Would I be right in saying though that the graphics card is at fault, I've heard that internal graphics cards can be more tempormental.