It hard to tell.
But it sounds like you got a CP2000 notice. Check the top, right-hand side of the letter.
Assuming that it is the case, the letter you sent is a "proposed" change to your tax return. Most likely the IRS recieved documents (W-2s/1099s) that did not match up to what was reported on your tax return. The proposed balance is adding in the missed documents into the tax return and recalculating the taxes on it. The letter should have provided which form was reported to the IRS but not on tax return.
If you agree, once you reply they will assess the balance and any refund you get in the future will be applied to it. If you don't agree, you can respond and provide information about why you don't approve of the assessment.
If you need copies of your tax return and/or you W-2s/1099s, you can call the IRS or go into one of the local offices and they can print you out a copy of them.