My fuel gauge works when you crank the car up and then after about 30 seconds on 2004 Chevrolet Malibu

This just started yesterday after I filled up my car and went to the store and when I came out and cranked the car up then the gauge went down to empty, what could cause this problem ?

Asked by for the 2004 Chevrolet Malibu
sounds like you need a new sending unit