Diagnosis
This is typically a hardware fault called a 'line defect'. It's caused by a failure in the connection between the TV's main board (T-Con board) and the actual LCD panel, or a fault in the panel itself.
How to Fix It
Before assuming the worst, do a soft reset. Unplug the TV from power for 2 full minutes. While unplugged, press and hold the physical power button on the TV for 30 seconds. Plug it back in and turn it on. This can sometimes clear a temporary glitch in the T-Con board. Also, try different inputs (HDMI 1, 2, etc.) and the built-in apps. If the line appears on ALL screens, it's a hardware problem.
If That Didn't Work
Gently press on the bezel of the TV near where the line appears (with the TV OFF). Sometimes a physical connection inside the panel can come slightly loose. If pressing makes the line change or disappear, it confirms a connection issue that requires professional disassembly to fix.
Heads Up
This is almost never a fix you can do with software or settings. If the line is persistent and visible on all content, it's a component failure. Contact Toshiba support, as this might be covered under warranty if the TV is relatively new.