Most important) Follow the IRS procedures for reporting a fraudulent return, documented here: www.irs.gov/uac/Taxpayer-Guide-to-Identity-Theft
IRS INFORMATION REFERRAL TO REPORT VIOLATION OF TAX LAW
You have not provided any details. If someone used your Social Security number and filed a tax return, your own return will be rejected when you try to e-file. You can still file your return, but you have to print, sign and mail it instead. The IRS will process it and sort out who should have used your SSN at a later time.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.