Flying, circling coloured dots on loop - Android 5.1 install fail.

[fearofbirds]

Hello,

I was running a custom ROM (Cyanogenmod) and decided to return to stock Android. I downloaded the latest files from the developer Google site and followed instructions on XDA forum to revert to stock. And now my phone (Nexus 5) has been looping those flying circles on a black background for 15 minutes. I'm assuming it's not normal for this to be happening.

Any help? Thank you!