Do you mean after the April 15 filing deadline? If you are getting a refund, nothing happens. You get your refund. There is no penalty for filing late if you get a refund. If you owe tax and file (and pay) late, you will probably get a bill from the IRS for penalties and interest for filing late and paying late.
If you cannot file your tax return by April 15 you can avoid the late filing penalty by getting an automatic 6-month extension, which gives you until October 15 to file. The extension is an extension of time to file, not an extension of time to pay. You still have to pay any tax that you owe by April 15. See the FAQ at the following link for more information about an extension, including how to file an extension in TurboTax Online.
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