Hi @somebodyu ,
Most probably the digitizer (touchscreen) has become faulty or there is a loose cable connection in the phone.
Here’s a video that shows how to replace the LCD screen assembly (includes the digitizer - complete assembly not two separately removable parts, LCD and digitizer) in the phone.
The video will help as it shows how to open the phone so that the screen cable connection to the systemboard can be checked that is is OK. If it is OK then the screen assembly will have to be replaced.
Search online for LG Stylo 3 Plus LCD screen to get results for suppliers of the part.
If doing the repair yourself seems too daunting, contact a reputable, professional mobile phone repair service and ask for a quote to replace the screen assembly.