iMac 27 Late 2015 - Black Screen After HDD Replacement

Try reseating the display connector, if it doesn't work, you may have damaged something in the process as you said that it is your first time, and this is a hard repair. Try plugging it into an external monitor if you have one. If you can't get it up and running try to put everything back together as original as you can and bring it into an Apple store and see what they can do as it must still be under warranty.