Each party remains liable for the full amount on a lease. If one party leaves, there may be liability to the other, depending upon the circumstances.
I agree with Mr. Hawkins and Ms. Smith. You personally will still be liable for the full amount of the lease but may be able to sue your roommate for half of that amount. Small Claims court is definitely the way to go for this type of case. Try to resolve the matter out of court first by sending a letter to her or making a phone call, and if that doesn't work, file a complaint.