No one wants to hear that their return has been rejected, but it's not the end of the world. Just keep calm and work through it.
If the IRS sends a notice that your Social Security number was already used, you'll want to review your personal details, like name, address, and SSN, to make sure everything is accurate.
If it is, you'll also want to make sure you haven't already filed a return this year. In the rare event that someone else has actually used your Social Security to file, you should contact the IRS to discuss your options moving forward.
You can find more info on the IRS website linked here Taxpayer Guide to Identity Theft.