This is most likely a hardware fault, not a menu setting. Based on the reports here, the screen’s position/orientation sensor or related microswitch in the articulating display assembly has likely failed, so the camera no longer detects how the LCD is rotated.

What to do:

• Check your manual/settings briefly to confirm auto-rotation behavior hasn’t been disabled, but if it worked before and suddenly stopped, hardware is the likely cause.
• If the camera is under warranty, contact Canon for service.
• If not, have a repair shop inspect it. Community experience here suggests the faulty sensor may be part of the screen assembly and may require replacing the whole LCD unit rather than a small individual part.

If the camera has no signs of impact damage, it can still be a genuine component failure.