Your question is unclear because the sentence [" She shrugged, not knowing."] is fine, as is, depending on the context.
It's not exactly abbreviated as in "What do you want?" "Potatoes." where the last sentence [fragment] abbreviates "I want potatoes."
IF one wanted NOT to 'abbreviate', i.e. to use more words and be more specific, one might say,
"She shrugged; she did not know the answer." ADDED: Or see Owlman's suggestion, above post #2.