Mine did this once also. Use your volume controls until it says start. Then press the power button. Booted up just fine for me.
I'm also completely stock, unrooted, locked boot loader. I haven't had the desire to mod yet. I was on WP7 for 18 months and just recently came back to Android. I'm enjoying the stock experience so far. Modifications will come soon enough.
Sent from my Galaxy Nexus using XDA
Current Phone: Nexus 4
Retired: GS3, HTC One S, HTC 8X, Galaxy Note, Galaxy Nexus, Lumia 710, HTC Radar, Focus, DVP, HD7, Sensation, G2X, Galaxy S, G2, Nexus One, HD2, Touch Pro2, N97 mini, Touch Diamond2, Touch Pro, iPhone, G1, Tilt, MDA, Dash, HP 6315, Treo 600
If you build it, they will flash.