Default Screen Won't turn on, flashing white/blue LED

Lately my Nexus 5 (running CM11) will occasionally turn allow me to turn the screen on, and the notification LED will start slowly flashing a whitish-blue color.

It even seems to resist me trying to reset it, as I'll hold the power button down for 30 seconds at a time and it won't reboot. After doing this 5 or 6 times eventually it'll reboot and then act fine.

Does anyone have any experience with this type of behavior or know something to try? It's been happening for a few weeks now so I'm not sure if it started with the latest M-build or is unrelated.