My daughter and her apartment roommate had an argument and the roommate took my daughter's keys and locked her out. My daughter went to the landlord and asked him to open the apartment and let her get her personal property and car, apartment, and post office keys while the roommate was gone and he refused. She is on the lease and had a check for $75 because of a clause in the lease that stated she needed that money for him to do it. She explained the volatility of the roommate and the danger to her if the roommate returned. He still refused to open the apartment. She had to wait and when she went to the apartment to retrieve her belongings she was attacked and assaulted. There are many witnesses to the unprovoked attack on my daughter and have stated they would give witness accounts. Do I have a case against the landlord for his lack of response to my daughters inquiries. I live in Missouri and could not find any precedent for this action.

We don't know why the landlord refused, anything from holiday inconvenience to rent owed or a history of bounced checks or cash required. Does the lease say he has to open the apartment, or within what amount of time?
Did she describe the big argument? Perhaps if she had just said she locked herself out...
Your daughter has a case against the roommate for stealing her keys and locking her out, in Small Claims. But she needs to show financial costs she incurred, not pain and suffering.
I don't see much of a case of negligence against the LL. LLs aren't responsible for the consequences of roommate squabbles, and courts don't want to hear about it.

Witnesses to the attack are not going to help. You need to prove that the landlord refused to let her in the apartment, even though she produced ID and the lease provides for this. If you can prove that, you should have a case against the landlord.

As Joy noted, there are many unanswered questions which could impact on this. Clearly her first course is to sue the roommate for an illegal eviction and for damages from the assault.

You have no case whatsoever. If your daughter is of legal age she might have a case againist someone but not you. The LL did not cause the attack and opening the apartment could have caused a different attack had the roommate been inside. Her only case is with the person who attacked her, did she call the police?

I'm going to have to disagree. Based on the OP's statement that the lease provided for letting the tenant into the apartment with a $75 penalty, the landlord was bound to follow the lease. Now there may have been reasonable circumstances to not follow the lease, but unless there were, I think the landlord might be held, at least partially, responsible for the consequences of that inaction.

But only the tenant has a case, not the OP. And the case hinges on why the landlord didn't follow the lease, if indeed he didn't.

I think you'd be hard pressed to establish a case/effect link with his actions. His refusal did not force anyone to do anything. Now, let's say, she had life saving meds in the apartment and was in need of them and he refused to help her by unlocking the door that would be another story. In this scenario his action would have a direct effect on her. In her actual situation he would have had no factual knowledge that not letting her in would cause he assault. I wish we had more info such as his reasoning and what the roommates said to each other as this escalated.