Diagnosis
The TV's operating system has likely become corrupted, or there's a memory/storage failure preventing it from loading. A failed software update can also cause this boot loop.
How to Fix It
You need to perform a factory reset via recovery mode. With the TV unplugged, press and hold the physical power button on the TV itself (not the remote). While holding it, plug the TV back in. Keep holding for about 20 seconds until the recovery menu appears. Use the volume buttons to navigate and select 'Factory Reset'. This will wipe all your apps and settings.
If That Didn't Work
If the recovery menu doesn't appear, or the reset doesn't solve it, the internal eMMC storage chip may have failed. This is a hardware fault requiring a main board replacement, which usually isn't economical.
Heads Up
This reset will sign you out of all your apps (Prime Video, Netflix, etc.) and delete all installed apps. You'll have to set up the TV like it's brand new again.