First, talk to the landlord about finding a replacement roommate. Remember that your lease agreement is a contract and you can negotiate new terms, if the landlord is willing to agree. If you know someone whom you want as a replacement roommate, get the landlord to agree to allow this new person to assume the responsibilities (pay the rent, etc.) and rights (deposit, etc.) of the roommate who moved out. Put it in writing! And get the landlord to sign it.
If you do not have a replacement roommate in mind, then try negotiating with the landlord to move to a smaller (cheaper) apartment that you can afford on your own, or with your remaining roommates.
If neither of these first two options will work for you, then you may need to consider breaking the lease. See the section at this site for information about what you need to do to give proper notice and what the law obligates you to pay – rent until the end of the lease term unless the apartment can be rented again; damages to the apartment beyond normal wear and tear; and the landlord´s expenses to rent the apartment.