Why would my gas gauge stop working after a fill up? on 1990 Buick LeSabre

My gas gauge was working fine. I went in to fill up my car and after I was done the gauge only read a half a tank. I know the car is full of gas. Do you have any ideas what it could be? Thanks.

Asked by for the 1990 Buick LeSabre
The in tank sending unit is most likely the cluprit!
Crap! Its going to be one of those days. Oh well. Thanks.
U welcom,, sorry!
At least it shows the tank is "half full" think positive!
If it works from that point to empty, it's better than not at all.