This bug affects many devices and is always better to let the device do its own thing or else you may face issues. You should wait for 15-20 minutes as migration may be going on below that Blank Screen.

Many times you see a blank screen once the installation is over.Try soft reset first and if it doesn’t work try hard reset. If gears are stuck then only issue arises. In case you see spinning gears for longer duration during update process, you should wait if gears are spinning.Try a soft reset and try downloading and installing the build again.Plug-in your device as in some devices there is a “Battery Saver” bug that stops download and installation of the build.Installation seems stuck at 0% for hours.Windows 10 Mobile Installation issues & workarounds: Here is your tutorial to prepare your devices and update to Windows 10 Mobile successfully for the best user experience. Here is the list of devices that can upgrade from Windows Phone 8.1 to Windows 10 Mobile. Not all devices are eligible to get Windows 10 Mobile upgrade. Windows 10 Mobile Build not being offered issue:

Cannot update viber windows 8.1