My initial diagnosis of my UE40D5520 which rebooted at the splash screen was that the capacitors on the power board had popped.
Apparently a common issue, and very easy to fix...
You can easily buy online cheaply and replace like for like any of the popped ones with a couple of solder points on each.
Take a photo first for their orientation, but usually they have a negative marking, and the short leg is negative too (double check though)