If the fuel gauge on the dash gives no reading at all, then check the following:
Check all the wiring at the rear of the gauge.
To access the fuel gauge from the back, open up the bonnet to access the rear of the dashboard.
Check wires are all plugged in and in order. Use the codes on the wires, against the chart below to match their location on the dashboard. (You can also download this chart below).
Check the wiring to the fuel level sender on top of the fuel tank.
If both are wired up correctly, use a multimeter to check for power.
If there is power then check to see if the fuel level sender is faulty.
Measure the resistance with Ohmmeter.
Check the fuse box.
Open up the fuse box under the bonnet.
Check the fuses to see if any have blown. Replace.