Switch the terminal off and on and try to connect again. Check that the SIM-card is properly placed in its slot.
Terminal does not read cards.
Switch the terminal off and on and try again. The chip card reader might also be unclean. Clean the reader with a cleansing card and try again.
Power is off.
Switch on the terminal by pressing the Enter-button for a while or by connecting the power cord. The battery might also be empty and require charging.
Download quick reference guide VX 520