Flashing Green Light on Tower

Hi, GeekPolice. One of my friends has been having computer troubles, so I told him I'd see what I could do to help. I brought his tower back to my place, unhooked my tower (after shutting it down from windows) and hooked his up. I used the power cord from my tower (along with all the other attachments - keyboard, mouse, monitor, etc...) and attached it all to his tower. Everything went smoothly. Until I tried hooking my tower back up. Now, when I plug in the power cord to my tower, I get a blinking green light. The tower won't even turn on at all. After I unplug it, it slowly blinks out. When I plug it back in, same problem.

His computer is an HP, mine is a Sony VAIO. Mine's roughly 5 years old, his is 6-7 years old.

I probably did something I shouldn't have. Did the cord somehow configure to his tower, and now it won't work with mine? Is there anything at all I can do to fix this? Will buying a new power cord help? Thanks.

So I was able to get a solid green LED light and power on the PC. I found some people experiencing a similar problem, and they fixed it by aiming a blow dryer at the part of the tower where the light was blinking. Sure enough, after roughly one minute, it worked. Only now, I'm having trouble with my monitor. When I plug it in to the CPU, I get the "No Input Signal" message on my screen, and it goes into power save mode. The monitor works fine with my friends computer/tower, just not with mine. Could it be an issue with the graphics card? Like I said, I have made no hardware or software changes before, or since, experiencing this problem. Thanks.