Acer Aspire 4520. Fan died so I had to replace. Broke it down, step by step, before installing new fan and reassembeled no problem. Didn't even have any left over screws! But now it won't power on at all. I've checked all the connections and everything is connected. ACK! Any ideas?

1) Did you use an ESD wrist strap, and connect the alligator clip to a good ground?
(Electro Static Discharge)

Your body carries Static electricity.
Static will fry out, (Short Circuit) the hardware components in a computer.
You may not even see it, or feel it.
Your computer will. POOF! No computer, expensive door stop.

Static is generated by the movement of your clothes as you walk, or move around.
Static is generated when you walk across a carpet floor.
Static is generated by touching an object that contains static electricity, and the charge is passed onto you.
Just some of the ways static is generated in your body. (Or passed onto your body)

I connect the alligator clip to the metal frame of an empty desktop computer case.
Better method is to connect to an ESD anti-static mat. (A little unnecessary IMHO)

A Processor is the most susceptible hardware component to static shock.

2) If Anti-Static Precautions were observed, perhaps take a look at this pictorial disassembly guide, and see if you can remember anything you may have omitted. Perhaps a cable didn't get hooked up. Granted it's for an Aspire 4530, but should be the same in disassembly.

http://aspiregemstone.blogspot.com/2009/05/dissembling-aspire-4530-full-guide.html

A) Power On switch/LED module get plugged back in?
B) CMOS (RTC) battery plugged back in?

Just some off the top of my head. Post back in a Comment, and we'll go from there.

Regards,
joecoolvette