I got a ACER AT900 ASPIRE X3960 that i just bought yesterday, and i hooked up all the cables for the computer and monitor and all power cords. The computer turns on but the monitor just stays black or says "Check Signal Cable" I don't really know what that means.. Please help

Try another monitor. If that works, download the latest drivers for your OS here... http://us.acer.com/ac/en/US/content/drivers
If the old monitor still doesn't work, here's some tricks I would try...
1. Verify the monitor's power cord is plugged firmly into the wall outlet and the back of the monitor.
2. Verify the monitor cable is firmly connected to the computer and the monitor.
3. Verify that you have turned the monitor power switch on.
4. If the power light on the monitor is not on, test the wall outlet (plug in a light or something you know works).
5. If the power light on the monitor is not on, try a different power cord.
6. If the power light is on and yellow or orange or flashing, but nothing displays, or the display is incorrect, borrow a monitor from a friend. If that works, your monitor probably needs replaced.
7. Also, you could try your display on a friend's computer. If it doesn't work, your monitor probably needs replaced. If it works properly on your friends computer, the problem may be with your computers video output port, motherboard video chip (or video card, if you have one), or drivers.

contact acer service center

Today's laptops and desktop computers are more user-friendly. As more people start use computers, operating systems, software and hardware accessories are coming with better instruction manuals and streamlined interfaces. Here are a few basic steps to hook up your computer no matter which brand you bought.

Connect the power cable to the back of the computer tower. The male end of this plug is often on the computer, but it is recessed into the casing. Once this power plug is connected, you don't want to plug it into an outlet until the entire computer is set up. Attach the monitor cables to the back of the monitor and then to the back of the computer tower. This cable will probably be the VGA type, which has a wide set of prongs in the middle and two screws on the outside. Be sure to twist the screws in to ensure the cable has a solid connection. Secure the monitor's power cable to the back of the monitor and then plug it into an outlet or power strip. Attach the mouse to the USB port on the back of your keyboard. The keyboard then plugs into another USB port on the back of the computer tower. Turn on the computer and the monitor.

If the computer powers up but the monitor doesn't display anything, make sure the VGA monitor cable is secured. If the status light on your monitor doesn't turn on, check the power cable connection. Check that the mouse and keyboard work once the computer turns on. Now you can hook up other devices, such as a printer or a web cam. A printer often can connect to a USB port and also may have its own power cable; a web cam usually just plugs into a USB or firewire port.

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So either the computer isn't generating a video signal, the cable isn't carrying the signal to the monitor, or else the monitor isn't capable of receiving a video signal.To test this, first try a video cable that you know is good, in case your cable is bad, If it works with the other cable, the problem was your original video cable. If it still doesn't work, put your original cable back, and hook another computer up to the monitor. If the monitor makes the same message, the problem is with the monitor. If the other computer works when connected to this monitor, your problem was that the original computer isn't booting (which could be due to several reasons I will list below), or the video adapter is broken.Possible reasons for the computer not booting: bad power supply, bad power button, bad hard drive, bad memory, bad motherboard component. If you determine that it isn't booting, unplug everything except the keyboard mouse monitor and power cord, to see if that makes a difference. If it boots after that, something you unplugged was causing the problem, plug them each back in one by one, rebooting in between, and when it stops booting properly, the last thing you plugged in is causing the problem.

The Acer Aspire L100 is a desktop computer powered by a special DC adapter and power cable. The computer's motherboard incorporates a fuse designed to prevent any power surges from damaging the computer. If the fuse burns out due to a power surge or for any other reason, the computer will no longer function. You can check the motherboard fuse for damage yourself, then take it to a trained technician for replacement if needed

Shut down the Acer Aspire L100 and turn the tower around.

Press the "On/Off" button on the front of the tower. If the computer will not boot and the fan does not turn on, the fuse may be burned out.

Unplug the power cord from the rear of the tower

Turn the tower on its side and remove the Phillips-head screws connecting the back panel to the Aspire L100's cabinet.

Pull out the motherboard assembly gently from the back of the cabinet. The internal fuse is located on the motherboard behind the DC power jack, which is to the right of the monitor jack

Let me know if this helped..

2 Press the "On/Off" button on the front of the tower. If the computer will not boot and the fan does not turn on, the fuse may be burned out.

3 Unplug the power cord from the rear of the tower.

4 Turn the tower on its side and remove the Phillips-head screws connecting the back panel to the Aspire L100's cabinet.

5 Pull out the motherboard assembly gently from the back of the cabinet. The internal fuse is located on the motherboard behind the DC power jack, which is to the right of the monitor jack.

Read more: Fuse Location on Acer Aspire L100 ' eHow.co.uk http://www.ehow.co.uk/how_7815704_fuse-location-acer-aspire-l100.html#ixzz1XUVwmxuA

3 Unplug the power cord from the rear of the tower.

4 Turn the tower on its side and remove the Phillips-head screws connecting the back panel to the Aspire L100's cabinet.

5 Pull out the motherboard assembly gently from the back of the cabinet. The internal fuse is located on the motherboard behind the DC power jack, which is to the right of the monitor jack.

Read more: Fuse Location on Acer Aspire L100 ' eHow.co.uk http://www.ehow.co.uk/how_7815704_fuse-location-acer-aspire-l100.html#ixzz1XUVwmxuA

You should have get two cords, if you bought the monitor and the cpu, if you just only bought the cpu then you must buy a new cord .
Here you can buy it. POWER CORD

check the power light, is it stay on with green. If yes, probably inverter or back light tube defective. If not, make sure the VGA/DVI cable connector or power cord is secure with PC. if everything is secure then probably the power board or main board defective.

hi there, try a 15 pin male vga to vga cable ...select vga mode on monitor

Hook up the vga cable to the vga port on the back of your computer. Usually it is blue. Hope this helps.

Hi,
Try other outlet, check the power cord if it contacted the monitor. If nothing happened, maybe it's factory defect, since it still under warranty, you can still change it.
Thanks