What are my options here?

I shared an apartment with someone on a standard lease agreement. We paid our security deposit and rent separately throughout the whole term of the lease. Upon moving out, the landlord issued her a check for the lump sum of the security deposit with both of our names on it. I have since not been able to get ahold of her to get my half of the sum. Her bank allowed her to deposit the check without my signature (unless she forged it). Who is ultimately responsible? The landlord for issuing one check even though it was paid separately? The bank for allowing her to deposit it without my endorsement even though my name is on the check? Or her for attempting to deposit it?