I like the first part of your logline, Matt Trinh ("When a maid discovers something trying to communicate with her behind a cellar door"), but I suggest adding an adjective for the maid (the main adjective of her personality).

Also, I think the second part of your logline ("she discovers a terrifying secret about her employers") is vague. I suggest adding more details (the protagonist's story goal and the stakes).

Here's a logline suggestion: "When a _____ (adjective) maid hears something trying to communicate with her behind a cellar door, she discovers a terrifying secret about her employers and tries to/attempts to/fights to/struggles to/strives to/sets out to/fights/battles/etc. _______ (goal of story) so/in order to ________ (stakes)."

I changed the first "discovers" to "hears" so you don't have "discovers" in the logline twice. Or if you want to use "When a _____ (adjective) maid discovers something trying to communicate with her behind a cellar door," here's another logline suggestion:

"When a _____ (adjective) maid discovers something trying to communicate with her behind a cellar door, she uncovers/unearths/finds out a terrifying secret about her employers and tries to/attempts to/fights to/struggles to/strives to/sets out to/fights/battles/etc. _______ (goal of story) so/in order to ________ (stakes)."