iphone 6 plus had gray screen and went black and won't turn back on

Have you tried holding down the home button and lock button at the same time for 30 seconds?

Your probably having problems with it because of ios 9.2.1 everybody seems to be having issues

Did you already try to connect in recovery mode, as described in this article?

If you can't update or restore your iOS device

More troubleshooting can be found in this help article:

If your iPhone, iPad, doesn't respond or won't turn on - Apple Support