The touch-screen on my Mio

The connection to the glass is usually through a series of small conductive rubber strips around the perimeter (like a gasket). If the pressure on the rubber is released slightly, it may not touch the glass or the circuit board on the other side. The pressure is held mechanically when the assembly is clamped together.
Liquids could short the rubber strips causing unpredictable display and touch screen operation. It would need to be dismantled and cleaned.
Another possibility is the ribbon wire to the LCD assembly is loose or even cracked so the traces are broken apart in the ribbon.
The LCD assembly is difficult to repair by dismantling. I wouldn't recommend it.