Hi @casava_chips
The flex cable connection circled in blue has been broken and also you inserted it the wrong way round.
Here's a composite image showing the broken cable from your posted image compared to the same point taken from an image in the ifixit Samsung Galaxy Tab S2 9.7 SM-T813 Battery Replacement guide.
It looks like it might be the Hall Sensor - Samsung p/n GH59-14472A but I'm not sure and I don't know why it would stop the tablet from starting.
You could try inserting it the correct way and see if the tablet starts. If it does then when the tablet is turned the display may not rotate or perhaps the display won't turn on when a cover is removed or opened etc. even if the desired setting has been enabled.
(click on image)
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
3