Android Nexus 6 Bricked? TeamWin splash screen turns on and off
If you installed/flashed from your SD card, try to use a new one with your stock ROM in zip form(including stock recovery). Or try a riskier method and try to use Odin3 but you need to prep all files before the screen turns on again, but make sure you have the right files(NOTE: you will have between 15-25 seconds to do this).