How do I fix a phone that is stuck on the boot screen?

Force Reboot
Hold down the power key for 8-10 seconds until the device either vibrates or powers off. Once the device is completely off, hold down the power key until the device turns back on.
Check for Overheating
Monitor and make sure the device is not overheating. If it is, then cool down the device before attempting further troubleshooting steps.
Check Updates
If the device is stuck on the boot screen, try checking for an update to the device’s operating system system.
Wipe Cache Partition
Wiping cache partition may help remove erroneous processes that are causing the phone to freeze on the boot screen.
Perform a Factory Reset
This should be a last resort because it removes all user information and settings.