This may be a long shot, but have you had an appraisal done on your house in the past year? This would be proof of it's current value. I agree with the others, contact the realators in the area first thing in the morning. They can do a simple search of the listings in your area and tell you what they sold for in the past few months and most of those listings can be sent to you via email. I know as we were looking into moving and were sent listings by email. Also, do go to the hearing and see if you can get a continuance as was stated before. This will give you time to get an appraisal as to the value of your house. This will probably cost several hundred, but may be cheaper than you paying the double rate on the taxes especially if your property is not worth any more than before the assessment.
Oh, and if they won't give you more time, there is next year that you can fight this. I know you will have to pay this year, but at least you can try to get them lowered in the future.
I am not sure about NJ, but here in Idaho our assessment notices come out in the spring and it tells us on the notice that we have "x" number of days to file a complaint. Then we go to a hearing. I am surprised that you were not sent an assessment earlier as in most states it is manditory that they give you proper notification which includes enough time to gather proof of your property value.
Good luck! Not fun during the holidays...