Was it exactly $500 less; i.e, that nice round figure?
Make sure you are comparing it to the correct figure.
Look at your Federal Form 1040, Line 21a for your Federal refund amount. Don't go by any total refund amount that TurboTax might have shown you at the Tax Home, which includes the sum/net of Federal and state together (if applicable). Does a state refund account for the difference you were expecting?
If you are receiving both Federal and State refunds, make sure which one you received.
Did you have any TurboTax fees that you chose to pay out of your Federal refund? If so, in addition to your product fees there is a $40 service vee ($45 for California filers), however that wouldn't explain all of a $500 difference, since your fees wouldn't have been that high.
If the IRS reduced your refund, you should get a letter from them in 3 weeks or so, but with the Coronavirus situation, correspondence may take longer. If there was an offset, there "might" be a note at the "Where's My Refund" tool:
https://www.irs.gov/refunds
