If you don't want to pay for a formal appraisal, I recommend looking at recently sold listings of similar homes in your neighborhood.
There were 4 sales in the MLS and they ranged from 50,000-84,000.
Have you tried to "claim" your house so that you can edit it?
Here is the link to allow editing your property.http://www.zillow.com/learnmore/EditingHomeFacts.htm
If you haven't already, make sure you know what goes into calculating a zestimate. It is only a starting point.http://www.zillow.com/wikipages/What-is-a-Zestimate/Also, remember that assessment values were determined almost 10 years ago and even then, there were a lot of mistakes.