First, check your refund status at: https://www.irs.gov/refunds
Once you know your official status, you can narrow down what might have happened.
Did you have your refund sent as a check?
It’s possible that it was lost in the mail or stolen. Either way, you’ll need to report the missing check and have the IRS start a trace by filing form 3911 https://www.irs.gov/pub/irs-pdf/f3911.pdf.
Once it determines the check was lost or stolen, the IRS will let you know how to proceed.
Was your refund supposed to go directly to your bank account?
The IRS is not responsible if you made an bank account information error on your return. You’ll need to contact your bank or credit union to find out what to do.
If you already contacted your bank or credit union without results, file form 3911 https://www.irs.gov/pub/irs-pdf/f3911.pdf.
The IRS will contact the institution and try to help, but the IRS cannot require the bank or credit union to return the funds.