Nokia Lumia 620 Touchscreen Problem Solution Ways

Touch screen has encountered any kind of water damage or moisture.
Phone’s touch screen is damaged due to any physical collision.
Touch Screen IC might be faulty.
Loose connection Nokia 620 touch screen connector with board.
Screen might be broken which also affected touch screen.
Dry soldering of touch screen on-board connector.
Connector strip might be damaged.

First Try Cleaning The Connectors Its Touchscreen
The Second Check Touchscreen Supporting Components Work … Number 1 VREG_L17_SENSORS 2.8V Number 2 1.8V VREG_L11_IO …. Number 3 Its Value 2.2K (Or Jumper) Pin Number 5 And 6 Directly To D4800

See The Image Below:
Nokia Lumia 620 Touch Screen Ways

Now re-solder on-board connector and check.