The instrument clusters have had a high failure rate on these vehicles. The dealers have extended the warranty coverage up to 7 years or 70K miles, whichever comes first from the initial delivery date.
The diagnostics for this are quite easy as long as you have the right equipment.
First, with a compatible scantool, access the instrument cluster module. There is a function for gauge sweep test. This moves the gauges all the way up and all the way down. If a gauge does not respond, it is faulty. The instrument cluster is only serviceable as an assembly and must be programmed using a factory scantool after installation. Usually an $800 repair total.
If the sweep test shows the fuel gauge moves all the way up and all the way down, you likely have a faulty sensor in the tank. Ohming the sensor at the connector is the next step. The sender should read a steady resistance value between 40 and 250 ohms, depending on the fuel level in the tank. These have also have had a high failure rate. They are available separate from the fuel pump assembly. Usually a $400 total.