My IQon says no video lnput. ?

Hi , my IQon desktop pc when i power it on it says no video input on screen , i have checked all the leads and they are all securely fitted. I noticed when i switched on the other evening it got as far as the setup/ boot page and then froze , but mostly it just says after i power on no video input, so i leave it alone for a while and try again and it fires up perfectly, i suspect that something is not performing as it should be , system is 3 years old.
please can any one help.