Two computers, One issue after another.

Hello!

First, I would like to thank those of you who read this and try to help in advance. I am currently getting my CompTIA A+ certification, and as a result my friends keep bringing me their broke down computers to fix, seeing as how it could be good practice I've tried to repair a couple of them, and ran into the same problem. The two computers in question are both E-Machines, but are not the same model, both run on AMD processors, but once again not the same model. The problem is this, when I plug the power in nothing happens. Truthfully, some things happen. The fans (both the power supply and cpu fan) kick on, I can open and close the disc trays, but that's it. Nothing pops up on the monitor, I can't even tell if their preforming the POST. I've tried swapping out power supplies, but that didn't change anything. I was only having this problem with one pc, but after installing some memory, and swapping out power supplies I'm having the same problem with both. Any Ideas will help, thanks again.