macbook pro won't turn on

i am a computer tech who knows next to nothing about a mac. i have a macbook pro that a guy wanted me to fix as the shop where he took it had it for a month with no results. i booted it from a usb with puppy and saw that the hard drive was not showing up. after some research, i deduced that the hard drive cable was bad, so i replaced it. i then booted into puppy again and the hard drive was showing up, so i fixed it.
he didn't have the disc, so i unplugged it and sat it aside until he got the disc. when i was going to return it, it didn't turn on. i hooked up the power, and the light turned green, then went to amber. the power light on the front will flash about every 8-10 seconds, like it is in standby. other than that, nothing. all i did was leave it sit for a few days. after about half an hour, the light on the cord turned green, and the battery leds show all green when i push the button.
i tried reseating all of the cables on the motherboard, took out the ram and switched them, trying each in each slot also, took out the battery, and removed the motherboard and replaced it. i cannot find anything wrong, and do not have any other compnets i can switch out. any suggestions? maybe some known issues, or can it get stuck in standby? i cannot even get to the white screen to boot from the usb.

Just for the heck of it...try an SMC Reset:

Intel-based Macs: Resetting the System Management Controller (SMC) - Apple Support

- Nick

didn't work. as i was doing this i realized that i had tried that also. this is wierd, i can't hear anything spinning up, or anything other than the power light blinking.

Would help enourmously if you provide model and noperating system details. Do you have a system install disc or is it OS X.7 and/or later? It is a pretty old MBP if it has the OS X.6 DVD. Did you try a different SATA cable when you changed the drive?

the model is a1278 and it had osx snow leopard on it, but i think he is going to buy mountain lion. the original thing i tried was a known good hard drive. when that didn't get recognized, i ordered a new cable and installed it with the original drive, which was identified. now i cannot get anything except one flashing light.