Get a lawyer that works on tenant issues and sue the landlord.
Your city , county or state may have a department for tenant/landlord relations and can solve your problem without a lawyer.
Sue them in small claims court if you want money damages.
Move. Your lease has already been broken by the landlord and you are free to leave if you choose. Keep proof, the lease and pictures of the sign that can be dated, in case the landlord does attempt legal action.
This last one option is the one I would take if I didn't want to stay there. I would rather just go and forget them.