Compare this year's return to last year to look for clues. Your refund is based on the taxes you paid in during the year (based on your income) along with any credits available to you.
Another post of mine may help you with determining what is best for you with your employer.
You may have some state credits or deductions that you no longer qualify for or may you changed jobs or even typed in the wrong amount for tax withheld. There are many factors.