How to deal with landlord who locked me out?

I rent a basement apartment in NYC which I suspect may be illegal. I am behind on my rent and my apartment flooded. The landlord said that he needed access to fix the leak so I moved out and am staying with a friend temporarily and gave him keys and permission. Then he changed my locks and is refusing to let me back in until I pay him all of the rent. Someone told me to call the police but I feel I should speak with an experienced lawyer first.