Mini powers off during boot
Backstory: I purchased a late 2014 Mini about a year ago and attempted to setup bootcamp with Windows 10.
Following numerous failed attempts I made an appointment with a genius bar. After several more attempts by three different geniuses over the span of 3-4 hours, they were finally able to get Windows installed.
Flash forward a couple of months and Windows pushes out one of its big cumulative updates. I try to install the big update and it errors out over, and over, and over.
After scouring Google for days and trying a variety of suggestions I decided to throw in the towel and give up on bootcamp altogether.
But, somehow during the process of removing the Windows partition, I also managed to destroy the macOS partition along with the recovery partition. (yes, I am an idiot). Feeling extremely frustrated, I stuck the mini on a shelf and moved on with life.
Flash forward to today. I removed the Mini’s hard drive, hooked it up to my iMac, and reinstalled macOS. Put the hard drive back into the Mini, fired it up, and … it won't boot!
The LED comes on, I hear the startup chime, but my display remains black. Then, after 20-30 seconds the Mini powers itself off.
Any ideas of where I should start? What is the most likely thing I screwed up?