Mac Mini (late 2012) No boot, heavy fans after upgrade
Were you ASD safe? If not there is a good possibility you had some static discharge that shorted the board or the memory. 

Actions to take:
Does it let you reset PRAM?
Try unplugging the Mac mini and holding the power button down for 10 sec. Apple and others may say you do not need to hold the power button down but I have found with some models or issues it help.

Check to see if HDD sensor is shorted/damaged.
If none of this works the you will need to start part isolation or bring it to someone who can. 
i.e.
Take out the RAM and see if you get an error beep. 

Good luck.