Contact the IRS as soon as you can to discuss this.  They will advice you on how to proceed, asking questions and requesting verification of your identity and then providing you with a pin to file your return.  You can also start the process by filling out the Identify Theft Affidavit and submitting it to the IRS by mail or fax.  Address and telephone number are located at the end of the document.   I would also recommend that you call the Identity Protection Specialized Unit, they are very helpful in resolving these types of cases.  See their contact information and times of operation below.

Toll-free phone number: 1 (800) 908-4490

Hours: Monday through Friday 7:00 a.m. to 7:00 p.m. local time (Alaska and Hawaii use Pacific Time)