Diagnosis
The TV is stuck in a standby or failed boot loop. This is almost always a power board issue where it's not getting the right signal to fully wake up.
How to Fix It
Unplug the TV from the wall. Hold down the physical power button on the TV itself (usually on the bottom right or left side) for a full 60 seconds while it's unplugged. Then, plug it back in and try turning it on with the remote. This forces a full power cycle and clears any residual charge that can lock up the board.
If That Didn't Work
If it's still a red light and black screen, the issue might be with the main board. Try a firmware recovery. Download the latest firmware for the 50A6K from the official Hisense support site onto a FAT32 formatted USB drive (8GB or less is best). With the TV off, plug the USB into the TV's USB port, then plug the TV's power cord in. It should automatically start the update process. If it doesn't, you might need a board replacement.
Heads Up
If you hear any faint clicking from the back when you try to turn it on, that's a classic sign of a failing capacitor on the power board. That's a component-level repair job.