Running test builds can be a tricky business. The latest release of Windows 10 Mobile comes with a warning: don't panic when it looks like your phone has bricked itself.
A post announcing the latest release (Build 10136) warns testers that the installation experience still has some rough edges:
At the end of the upgrade the Lock screen will appear frozen without the date and time for up to 10 minutes. Please be patient and let it sit — eventually the date and time will appear and it will allow you to unlock your phone. There is a post-upgrade migration status screen missing, which normally would show you that it was making progress. If you get impatient and restart or mess with locking/unlocking your phone, you'll wind up in a funky state — so just relax and let it do its thing.
Announcing Windows 10 Mobile Insider Preview Build 10136 [Blogging Windows]