DIAGNOSIS This is a hardware fault, specifically a defect in the panel or its internal connections. A persistent vertical line is not a software or settings issue. It indicates a failed column of pixels or a connection issue along the panel's driver board.
FIX There is no user fix. You must initiate a warranty claim. Document the issue with a clear photograph. Note the monitor's serial number (found on the back label or in the OSD under General > Information). Contact LG customer support directly or your retailer. The 27GN950-B typically has a one-year parts and labor warranty. Be prepared to provide your proof of purchase.
IF THAT DIDN'T WORK Before contacting support, perform a full factory reset to rule out a corrupted firmware setting. Press the joystick. Go to General > Reset. Select Yes. This will not fix a hardware line. If the line disappears during the reset screen but returns in normal use, it is still a panel fault.
HEADS UP Do not attempt to disassemble the monitor. This will void the warranty. LG's standard process is often a board replacement or a whole-unit swap via an authorized service center. The repair time can be several weeks.