Warning! Major spoilers for The Haunting of Bly Manor below.
The Haunting of Bly Manor drops hints throughout the first half of the season that Hannah was dead all along, and her visions of cracks in the wall is a persistent indication of this fact. The most heartbreaking of the many clues regarding Hannah's death was the housekeeper's continued questioning of said cracks, especially since nobody else could see them. The crack is distinctive, and seems to follow Hannah by appearing in multiple locations throughout the house.
One of the cleverest and most heartbreaking twists of Bly Manor was Hannah Grose's death. Throughout the early episodes of the series, something appears to be very off with Hannah. She always refuses food and drink, never sleeps well, and her mind is constantly drifting off. Appearances of a crack in the wall that no one else can see plague her as well. As Bly Manor episode 5, "The Altar of the Dead", reveals, everything mentioned above are clues that Hannah is dead.
Hannah may be the most unusual of the many ghosts who haunt Bly Manor, as she is the only one who does not realize she is dead. However, all the clues presented were her mind's way of attempting to wake her up to the fact that she died, with the crack in the wall being the most pressing of all, given what it represents in the larger scheme of things. Hannah's subconscious protects her from this fact, since she's not ready to let go of the life she could have had - and wanted to have - yet.
The Crack in the Wall is the Last Thing Hannah Saw Before Her Death
A crack in the wall was the last thing Hannah saw before her death. Bly Manor reveals that Miles, while he was possessed by Peter Quint, pushes her down a well immediately before the events of the Netflix's series first episode; it happened mere moments before Dani's arrival at Bly Manor. As a result of the fall, Hannah breaks her neck and dies; the last thing she sees before she fades away is a crack in the wall of the well. The fact that she continuously sees that same crack in walls around the house is her subconscious screaming at her to accept her fate.
Hannah refuses to wake up because she can't let go yet. In fact, that's why she is the only ghost in the manor that every living resident of Bly Manor can always see. Her extreme dedication to her loved ones keeps Hannah tethered to her earthly life despite having passed on. It's that determination that allows her to manifest physically and go about her daily life in a nearly normal way. No one realizes she's a ghost until Dani's sacrifice in The Haunting of Bly Manor's ending releases her spirit from the grounds. These clues about her death are a testament to Hannah's character. She loves those close to her so fiercely that she finds a way to stay with them, even after her tragic death.