The state could have made a change to your return that has impacted your refund, or they could need more information to finish processing part of your refund. Another possibility is that it was partly taken in order to offset a prior debt owed.

In all of the situations above, you will receive a letter explaining exactly what happened, what additional information they might need (if any) and contact information. You could also call your state tax department before receiving this letter, they might be able to access your account and explain what happened.