You don't need to go back to an earlier save, sorry for the confusion.
To enter her room for the diary, you need a surveillance cam and your repair skill needs to be at least 1 (do the bulletin board quest in the hall, then you can repair a cam on the safe in your room).
First, try to get into Naira's room in the afternoon while you have a cam in your inventory.
Next, visit Esther in the garden (in the afternoon as well) and ask to get into Naira’s room to fix her PC.
If you wish to re-enter without asking Esther, you can grab Naira’s spare key in her closet.
I'll be modifying this a bit in the next version to make it less complicated