About how long has it been since you filed? Did you efile, or did you print, sign, and mail the return?
If you efiled, double-check with this tool to be sure the return was efiled successfully and accepted:
https://shop.turbotax.intuit.com/efile/efile_status_lookup.jsp
If that showed it was accepted (or if you mailed the return), what do you see at the IRS website below for the status of your Federal refund?
https://www.irs.gov/Refunds
NOTE: When entering your refund amount, use only the Federal refund amount and not any total refund amount that includes a state refund. The figure is shown on your 2018 Federal Form 1040, Line 20a.
If that IRS tool shows that the refund has been sent, but you never received it, report back for more info.
If the IRS tool says it is still "being processed", and if it's been longer than 21 days, we can tell you how to speak to a live agent at the IRS to see if they can tell you anything about the delay.