1. Go to Safari / Preferences then select the Autofill tab. Make sure all three boxes are selected.
2. Try using a different browser. Reason being, some sites work differently with the autofill feature on different browsers.
If you don't have another browser installed, it's good to have one for situations like this and in case Safari isn't working, you have a backup browser.
I go back and forth between Safari and Firefox all the time.