Get a job. Get a second job. Sell blood plasma. Sell anything you own of value. Borrow from friends or family. Take in a roommate. Move out before you get evicted.

IF you have a plan (not a fantasy, but an actual plan) for when and how you would be able to pay your rent, contact your landlord before you're late, explain the situation, tell him your plan, and ask him to work with you. If you take this route, make absolutely sure you stick to that plan and pay him when you said you would.