Deadgirl

Rickie and J.T. are two high school seniors who gaze at the girls they wish they could get, especially Joann, the object of Rickie's affection, whom he has known since he was a child. One day, they decide to cut class and end up in an abandoned psychiatric hospital. They discover a mute, naked woman in the basement, chained to a table. While J.T. is interested in raping her, Rickie refuses and leaves. J.T. confronts Rickie the next day, and the two return to the basement where J.T. reveals that the woman is undead, which he discovered after attempting to kill her three times.
Rickie finds that J.T. invited their friend Wheeler to rape the woman, nicknamed "Deadgirl", as well, and decides that it is time to free her. He is able to cut the chain on one hand free before he hears J.T. and Wheeler approaching. He runs to hide, and J.T. begins to rape the woman. After he notices that her hand is free, the woman attacks him and scratches his face.
During gym class one day, Rickie asks Joann out on a date, knowing she has a boyfriend. She rejects him, and that afternoon, Joann's boyfriend Johnny and Johnny's friend Dwyer beat up Rickie and Wheeler. Wheeler rebuts that they "have their own pussy now" and don't need to hit on Joann. Johnny throws Rickie and Wheeler in his trunk, and he and Dwyer drive to the asylum to see Deadgirl. Rickie convinces Johnny to force Deadgirl to perform oral sex on him, and Deadgirl instead bites Johnny's penis, infecting him. The next day, Johnny races to the bathroom during class and his intestines burst out of his body, leaving him in the same undead state as Deadgirl.
Having figured out that this is an infectious, rotting disease, J.T. and Wheeler decide it is time to make a new Deadgirl with a fresh body. They lie in wait outside a gas station for a female victim. After an unsuccessful kidnapping and recovering from their beating, Joann shows up to confront them about Johnny. J.T. and Wheeler capture her and take her to the basement. Rickie heads to the basement with a machete to free Deadgirl and finds Joann and Deadgirl tied up to each other, encircled by J.T. and Wheeler. J.T. tries to convince Rickie to let Joann be bitten while Wheeler starts to feel up Joann. Rickie defends her by slicing Wheeler's hand off while Joann unties Deadgirl who feasts on Wheeler, then J.T.
Rickie and Joann flee through a stairwell door but cannot escape through the locked exterior entrance at the top of the stairs. Rickie runs off to find an escape route, and when he returns, Joann is not where he left her. Rickie returns to the basement and finds Deadgirl, who is about to attack him. She runs into him, knocking him down, breaks open the door and escapes to the outside. Rickie then sees Joann, who is about to collapse. He catches Joann as she falls, then sees that J.T. has stabbed her in the back. J.T. urges Rickie to let him bite her so she will live as undead. Rickie holds her for a moment, assuring her that he loves her and will save her. She coughs blood into his face, tells him to grow up, and asks for help. Later, a cleanly dressed Rickie lives normally as he walks outside in the direction of the asylum. There, in the basement, surrounded by soft lights, tied lying in bed wearing clean lingerie, is his Deadgirl, Joann.

Question: Why Rickie and Joann cannot escape through the entrance at the top of the stairs?
Answer: It was locked.