Commonly reffered to as 'The 3 bars', three horizontal lines appear in the gear display.
This indicates a shifting system fault, or a gearbox malfunction.

It can be caused by anything from a sticking gear, to a sensor spike.
The first and most obvious thing for you to try is switching off the ignition/disconnecting battery and try restarting.
If this fails you may want to look at the following possible resolutions.

Put the car in what would normally be 'reverse', and leave the handbrake off.

Pushing on either the front 'A' pillar or 'B' rear pillar, rock the car backwards and forwards.

You may hear it finally drop into gear, or the car will suddenly start beeping as the gear selects gear '0'.
When you next press the brake, or move the lever, it should now function correctly.

The second option to look at is the gear position sensor.
You can remove the wheel for more access.

Firstly, check the cables are not chaffing or worn.
If they're okay, then you can remove the gear selector using an E10 socket.
A replacement part is going to cost around £60 from smart.

The other option for you to try is to remove the gear selector motor.
Put the car into what should be 'neutral'.
Sometimes the gearbox can 'stick' in gear, jamming the motor.

The 2 securing motor bolts can be removed using an E10 socket.

This motor will rarely fail in it's job, so you should be able to remove the motor and hear the gears 'spring' back to neutral.
You can now refit the motor.