What would be the most common reason for the fuel gauge to not work?

most likely the sending unit ,float arm/switch has gone bad.you can just replace that part for $138.99 or $161.99(pump NOT included)..there are two diff models..or replace the whole assembly(pump included)for $232.99..or just leave it like it is and use the trip button on odometer to keep up with your miles so you'll know when to re-fuel...this is if you do the work yourself. if a shop does it,price will be around $574.00--$959.00,depending on how hungry the shop is..there is a slight chance the gauge is broken,but will require a gauge sweep test to verify? i'm a betting it's the sender!

As you drive the vehicle , does the GAUGE respond appropriately for the time/miles driven , despite what the ECON display says?