Can't get a signal with my QA-1800

I have just bought a Sharp QA-1800 the problem is I can't get a signal. I have connected it to my PC using the 26-pin/15-pin lead but all I get is a message that says "No-Signal (P)" Am I missing something, you see I have onlt had one projector panel before, whicj worked fine until the PSU burnt out, but I have never had a sharp LCD Panel. So if you are familiar with these panels can you please help me out. I'm pretty much a newbie at this and would greatly appreciate any advice.
Oh and I know that the LCD panel works bescause the menu is working so there is definately power going to the Panel, just no signal.
Thank you.
Thanks for the reply. I will try out these suggestions. Do you know what the coeerct resolution should be for the PC, and the refresh rate?
Also, if the PC settings are the problem, why can I not get a signal from the composite video which has nothing to do with the PC display? I had no problems getting my VCR and TV to work with my old LCD panel and its the same VCR and TV I have now, but I can't get them to work with this panel.