Each apple computer is registered within apples system with the exact specs that it has when shipped. If you change any of these we know right away. Also after seeing and opening thousands upon thousands of these we know what its like to crack open a fresh one and what it is like to crack open one that someones had their hands in already. There really is nothing you can do. That being said Apple doesnt usually get too upset about upgraded RAM or HDD. At a certain point they usually just assume that a licensed third party has installed them. DO NOT send it to apple that way though. They will replace any parts they find to factory condition before sending it back as an insurance for their work. They will toss any upgraded HDD or RAM. It isnt pretty i have had people really angry for that but Apple has to send it back as the system was specced out of the box.

What does the light in front do? Is it solid when you turn it on? Is it pulsing slowly like it would be in sleep mode? That little light is actually an indicator for us techs. If its a solid light see here: http://www.ifixit.com/Answers/View/98080...

There is no great way to troubleshoot this sadly that i have come up with. My first thought is if you have another computer to check your router, if it is still functioning its hard to think that it wouldnt fry up that thing yet it would fry up the computer. Not saying it isnt possible because i have seen things that are a bit more odd. That being said if you had possible lightning hit your house it could have done damage all over because as we all know electricity likes to jump and jump far. The Mac mini that you have has an external power supply this is a plus in this situation because it may be easy enough to take it in to a local Mac dealer and see if that cant just plug it in for you and see if it powers on. If this does not happen to work yes you may have harmed it beyond what i would recommend repairing. If it is not the power supply it is bound to be the logic board. The logic board of a computer really only has Five main functions and one of those is distribution of electrical power. IF you did get a...

More often then not this is a problem with the logic board. What is happening that you are seeing is the computer is failing the power on self test (POST) and that will make it refuse to boot up. One thing you may want to try is opening the computer back up and making sure all of your wires are connected the way they should be. That being said 9 out of 10 times i have had this issue land on my bench its been the logic board. IF you pulled the computer apart with nothing to eliminate ESD you may have harmed your own logic board. I actually just had a computer with the same issue come on to my bench last week and it was sure enough a failed logic board. You may have shot yourself in the foot because you opened it yourself unless you are an ACT but for a while apple was honoring a repair of the computers with that issue that was caused by a faulty GPU. Off the top of my head i cant say if they are still honoring that or not.

Nevermind my last post about what kind of macbook. I am just gonna assume its a MacBook Pro since you said "latches". This usually happens because of the battery swelling and pushing against the sides of the slot where the battery is. In the past i have solved this issue by pulling both the tabs at once and taking a spudger which is sold on this site, you can also use anything that is a dull plastic (do not use anything sharp it is a battery and that could be dangerous) and pry in between the case and the battery. The side of the battery nearest the latches with be the part you want to pry on which i assume you know since you said you have replaced the battery before. Let me know what you get.