If you’ve had more withheld than you owe, then you probably have nothing to worry about. You’re simply delinquent in filing for your refund. (If you owe, then there’s a penalty for filing late.) Your employer sent the IRS about what the IRS thought you would owe by the end of the year. In my experience, that number is always more than what you actually owe based on your adjusted gross income. So probably you are owed money. (This is assuming you were paid above board and that you claimed the “proper” number of exemptions on your W-4 or whatever.)
One place to find out what you’ve earned that’s on record is the Social Security Administration. They can provide you with your “total income” (again what is on record) for every year that you’ve been employed.
irs.gov is another resource, which includes a list of vendors who will let you file online for free.
Not to go dark side of the force on you, but I also recently finished reading an interesting tomb on the issue called Cracking the Code (available cheaper elsewhere), which purports that we are tricked into believing we owe on an unconstitutional tax through a giant apparatus (the IRS) of obfuscation, intimidation and manipulation.
Sadly, Astro, the author confirms that the income tax does apply to mailmen and other Federal employees, but to no citizen who does private, non-Federal business.