How did they get enough info to steal my tax return?

I contacted your company in April of 2015 as soon as I was informed my tax return had already been filed. Some how despite the fact that my joint return is 64+ pages in length, when we electronically filed in April we received a written letter from the IRS informing us that a return had been filed using our names and SSN's. Since then we have filed and received our refund however, if someone has this much information, what do we do to protect ourselves? We have been with LifeLock through AOL and what was offered after the fact was you wanted us to turn over POA so all the correspondence would go through you instead of to us directly. I did not fill out the POA paperwork because I figure if they had acquired all this information while you were on the job, what good is it closing the door after the cow has runaway. I have not received any special code or number from the IRS so I am unsure if this will reoccur in 2016.