If he locks you out without a court order so authorizing, call the police. Theoretically one tenant does not have "standing" to terminate another tenant's tenancy because that is the landlord's perogative, not a tenants.
You can certainly explain the situation to the landlord; if you also pay the rent that is due and owing they are much less likely to evict you. Yes, you are a tenant if you live there, there is no legal requirement that you be "on the lease". If the tenant told you they were not coming back arguably they may have abandoned their tenancy (failure to pay rent coupled with affirmative words or conduct indicating an intent NOT to resume the tenancy).
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I agree with Ms. Powell and would suggest you approach the landlord with the rent in hand and try to negotiate a new lease. The landlord may not know anything about the situation and you should try getting on the lease and get your old roommate off the lease. Right now you may not have any rights in the lease but you might also not be responsible for the past due rent. Either way it is time to fix this issue.
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