diputing a judgement

A few years ago I rented an apartment with no lease from an older lady. While I lived there, she passed away and the estate became her family's property. They were most interested in having me leave due to the fact that I paid very low rent for the apartment, however rather than just ask me directly to move they threatened to evict me over noise problems. Anyway, it turns out now, almost two years later their is a judgement against me on my credit report for $666 which I don't have any knowledge of. I never had unpaid rent and I left the apartment in good condition. What can I do, this is really messing up my credit.