Q: iphone 6 stuck at apple logo, start up, **** down and again and again

Try placing your phone into Forced Recovery Mode and restoring through iTunes

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support

If it won't allow you to put it into recovery mode, because of the continued rebooting, it may be a hardware issue that will need servicing.

Contact Apple for support and service - Apple Support

Try the steps here to see if they help.

If your iPhone, iPad, or iPod touch won't turn on - Apple Support