IRS DEBT

I was told by the IRS that I owe over $4000 for the year 2006.  I was told 2 different stories over the phone as to why I owed this.  Then, to top it off, I just got a letter from them saying no return was even filed for 2006.  So, I have no presentment, but have been threatened with a levy.  I'm an SPC.  Should I just go ahead and start a CAFV?  What would be the best way to proceed?