gas is not showing up after filling it

What is your question?

I have a 2008 ford focus se. A few days ago my bf and I went to fill up my gas. After we did we noticed that it did not register on the gauge and the miles to empty. He then tried to reset the gauge by undoing the positive cable on the battery and reconnect it. That seemed to work but he wanted to try and top it off again and see. It then did the same thing and he fixed it the same way. What could be causing it not to register unless it is reset?

How long has your 2008 Ford Focus had this problem?

less than a week