Are all of the cables plugged in correctly?
Yes. No.
Are there any lights on the Tower / Monitor?
Yes. No.
Is the computer beeping, or have a red light?
Yes. No.
The RAM inside is not properly seated, or is defective. Send the RAM reseating video from YouTube..
If the customer is able to, try to boot the PC with another known-working monitor..
The PC powered up and booted to Windows.. The PC still isn't displaying a picture, but has lights..
The Monitor is defective, offer to replace if they're under warranty..
Test with another VGA cable. Is the PC able to display images on the screen?
Yes. No.
The VGA cable is bad, offer to send a replacement..
There is likely some type of hardware issue with the PC tower itself. Offer to set up repair, if under warranty..
Test the Tower / Monitor with the other power cable, and if necessary, in another outlet. Verify the outlet works with another device such as a lamp etc..
The other cable in same outlet allowed it to power up.. It powered up in another outlet.. It still won't power on..
Offer to send a replacement power cable..
The customer's power outlet is bad..
Offer to set up a repair for a failing power supply / monitor..
Connect the power cable, ensure the video cable is plugged in to both the PC and the Monitor, and the mouse & keyboard are plugged in. Press the power button on both. Did the PC power up?
Yes. No.
Verify it boots into Windows..
Test the Tower / Monitor with the other power cable, and if necessary, in another outlet. Verify the outlet works with another device such as a lamp etc..
The other cable in same outlet allowed it to power up.. It powered up in another outlet.. It still won't power on..
Offer to send a replacement power cable..
The customer's power outlet is bad..
Offer to set up a repair for a failing power supply / monitor..