"Is she at home?"
Translation:Er hun hjemme?
'hjemme' is the preposition (well, technically an adverb), and it means 'at home'.
(Historically, it was considered the dative form of 'hjem'. However, since dative is no longer in use, it has been recategorized as an adverb.)
You can say "i hjemmet" if you're talking about homes in general, but this is not a common expression.
"De fleste har bøker i hjemmet" = "Most people have books in their homes".