what should I do??

Did you file your taxes for 06 but just not pay them, or you never filed for 06?

You should definitely file 07 taxes before April 15 or else they charge penalties for late filing.

Your best bet is probably call the IRS (find your local office at IRS.gov), and ask them. From what I've heard from the others, the IRS is easy to work with and they won't treat you like crap. They'll probably just tell you to pay for last year, and may charge you some interest on it.

I did file my 2006 taxes but did not pay them yet. I actually DID send them a check for the money I owed them last year but it must have gotten lost in the mail because it never came out of my bank and they did send a few letters since then. I do plan on filing my 2007 taxes soon but I wasn't sure if I should pay from 2006 first.

If they haven't processed the payment and they are still saying you owe them, you probably should stop payment on the first check and write them a new one. You could even drop it off at your local IRS walk-in office to make sure they got it.