I plugged the HDMI connector and turned on the computer, but when the log in screen appears the laptop screen appears split in 2, one top and one bottom. The external monitor connected via HDMI works fine. I just need to know how do I disable dual screen from laptop and back to normal.
I assume you want to use the external monitor. But regardless, all you have to do is go into Display, select the monitor you want to turn off, & turn it off using the slider below the display image.