You do not say how or why you are pretty sure someone changed your password or filed a tax return using your information. Are you sure you are in the right account? What makes you believe that someone else filed using your account?
Many people have multiple TT accounts and forget how to access them. Log out of the account you are in now.
https://ttlc.intuit.com/questions/1901535-forgot-your-turbotax-online-user-id-or-password
https://ttlc.intuit.com/questions/1901486-how-many-turbotax-accounts-do-i-have
If in fact someone else used your SSN on a tax return, you should go ahead and prepare and file your own tax return, but file it by mail. That way, the IRS will process your return and sort out the duplicate use of your Social Security number.
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.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.