Depends on facts not posted--(what kind of lease, what terms, what agreement was made with the roommate....) you may have to sue in small claims to recover damages. LL may sue you if you cannot make rent. Recommend you find new roommate acceptable to the LL immediately, then take steps to recover damages from the former roommate.
READ THIS BEFORE CALLING OR EMAILING ME: I am licensed to practice before the state and federal courts in Virginia. We have not established an attorney-client relationship unless we have a signed representation agreement and you have paid me. I am providing educational instruction only--not legal advice. You should speak with an attorney to whom you have provided all the facts, before you take steps that may impact your legal rights. I am not obligated to answer subsequent emails or phone calls unless you have hired me. I wish you the best of luck with your situation.
I agree with Attorney Rafter, the lease may control your options. You may wish to consult with a local landlord/tenant attorney who can review the lease and facts of your case to advise on your best course of action.
My answer to your question does not create an attorney-client relationship. Furthermore, I have not reviewed any laws or cases related to your question in formulating my answer. I am simply providing my "gut reaction" based on the facts as you described.