No Power after replacing Hard Drive

I'd take out the battery while testing, and then reseat the RAM. What color is the light on the AC adapter plug? With the battery out, it should be solid green. If there is no light, if it's dim green, or it flickers, that's a problem and could point to a problem with the DC-in board, or other related connections. Or, you AC adapter might have chosen to coincidentally flake out simultaneously with putting in the new drive, so testing it with another AC couldn't hurt.

I would keep the machine opened up, and try powering on while applying pressure to the topcase's connection to the logic board. There is enough slack in the cable to do this, and it makes life easier to keep the topcase disconnected until you know it powers on.

If the AC adapter light is green, and none of this works, I'd look into jumping the logic board. I believe this model has two connectors on the board you can use jump the power (ruling out the topcase as the problem), although I'm traveling an I don't have access to the service manuals in order to look up the location on the board.

As Charles points out, hard drive replacement should not cause a power issue. You might want to remove the hard drive and drive cable entirely while testing...any MacBook should power on just fine to the blinking question mark while its hard drive is out.

Hum, you should not be having power issues just from replacing a hard drive only. You may have to go a little deeper to find out what went array, so make sure that you have the power connector plugged up and it shows good. Now if this reveals nothing then remove the battery and press the power button for 3 seconds. if you have the top cover off look at that connector cable which connects the I/O board to the main logic board (you can see it underneath just a little when you look right along the area where the hard drive connector snaps on the board). The only way to gain full access to that particular cable is to remove the I/O board itself and lift it up and disconnect it from that I/O board without pulling it out of it's socket from the main logic board (YOU MUST BE CAREFUL WHEN DOING THIS). I would think that this would not be the problem but it's something that you may have to take into account. The advice you get from this site greatly depends on the accuracy of what people post. P.S., it is possible that you could have damaged that power button ribbon without knowing it. To much bending it right at that plastic connector can cause a break with in that ribbon without EVER realizing it.

Check the battery again, remove it and put it back in again, it's very easy to mess something up just slightly.