Monitor not working on tower but works on other towers.

[Killerken3000]

I need help.
First, i haven't used my Computer about 1 year, and when i use it My Monitor no signal shows up and goes to sleep. I tried to do some testing and had 2 hp towers, 1 hp monitor/1 samsung monitor, 2 hp laptops/1 toshiba laptop. I hooked up both towers to a monitor, and no signal so i switched both monitors and still no signal. Then I hooked up all laptops to both monitors and it worked. When I hooked up the towers again still no signal.
Do you guys have any suggestions on how I can fix it?

 

[Milk_The_Elephnt]

do the Towers have graphics cards or are they on board graphics? or both?
are you sure the monitors are set to the right input?

 

[MindoverMaster]

What types of laptops or desktops do you have?

 

[Killerken3000]

I have 2 Hp laptop/1 toshiba laptop and 2 hp towers

 

[Killerken3000]

the Towers have graphics cards

 

[Milk_The_Elephnt]

OK, so let me get this straight, you have two monitors, neither of which are working on either towers but both monitors work with all the laptops?
try switching the graphics cards between towers?

also, the model numbers/name of the towers would be useful.