A ossibility is that your refund was reduced by the government, either to correct an error or through a refund offset. The IRS Where's My Refund tool will let you know if your refund have been offset.
You'll get an explanatory letter from the government if your refund differs from what was on your filed return.
Federal and state refunds are sent separately, each on its own timetable, so you may have gotten one but not the other.
