If the LCD display controller is still working then sending it a SETLCD 0,0,CHR$(1) would clear the screen.
If you reset the PLC by executing a "RESET" command or [MaRST] ladder function or by clicking on the RESET button on the online monitoring it will also issue command to reset the LCD display.
But if the LCD controller hang (due to noise coupling on the LCD cable) then there is not much you can do except to power on reset the PLC which will also power on reset the LCD display.