I have received a letter from the Colorado Department of Revenue stating that "Additional Documentation is Required" and that my return cannot be completed. I need help.

It says the payments you are claiming do not match the payments we have on record. I've just reviewed my Colorado Individual Income Tax return, and I'm not claiming any payments at all. I have no idea how to resolve this, as the letter only tells me how to submit the requested documentation (copies of checks or money orders or confirmation of electronic payment). They say I have 21 days to comply or I will lose any credits, subtractions, or income tax calculations that cannot be verified.

Aside from calling them next week, I don't know what to do next. Any suggestions or advice?