Diagnosis
The TV isn't recognizing the HDMI input, but the port itself is rarely the first culprit. It's usually a handshake issue or a setting that got flipped.
How to Fix It
First, make sure you're on the correct HDMI input. Press the 'Input' or 'Source' button on the remote and cycle through. If you see the Fire Stick menu flash for a second then go black, that's the handshake problem. Go to Settings > Device Preferences > Inputs. Find the HDMI port you're using (like HDMI 1) and change the 'HDMI Format' from 'Standard' to 'Enhanced Format'. This enables the full bandwidth the stick needs. Also, try a different HDMI cable if you have one.
If That Didn't Work
The TV's HDMI-CEC (called 'EZ-Sync' on Hisense) can sometimes interfere. Go to Settings > Device Preferences > Inputs > EZ-Sync and turn it OFF for that specific port. Unplug both the TV and the Fire Stick for a minute, then plug the TV in first, wait 30 seconds, then plug the Fire Stick in.
Heads Up
The 50A6K only has HDMI 2.0 ports, not the newer 2.1. If you're trying to pass through a high-frame-rate 4K signal from a next-gen console, you might hit limits that cause black screens. Stick to 4K@60Hz max.