DIAGNOSIS: The TV is getting power and the main board is trying to boot, but the backlight system isn't firing up. On a Mini-LED set like the U7K, this is often a power board issue, not the actual LEDs.
FIX: Unplug the TV from the wall for a full two minutes. This lets the internal capacitors drain completely. While it's unplugged, press and hold the physical power button on the TV (if it has one) or the joystick control on the back for 30 seconds. Plug it back in and try turning it on normally with the remote.
IF THAT DIDN'T WORK: The temporary 'soft reset' didn't clear it, so you likely have a component on the power board that's failed under load. This isn't a user-serviceable fix; you'll need to contact Hisense support for warranty repair or a technician to replace the power supply board.
HEADS UP: Don't be tempted to open the TV yourself looking for a blown fuse. These TVs hold a dangerous charge even when unplugged, and you could easily damage the delicate Mini-LED array.