lost has 14 distinct synonyms spread across 6 of its 10 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 3 recorded opposites are found, saved, won.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
no longer in your possession or control; unable to be found or recovered
“a lost child”
having lost your bearings; confused as to time or place or personal identity
“I frequently find myself disoriented when I come up out of the subway”
spiritually or physically doomed or destroyed
“lost souls”
deeply absorbed in thought
“as distant and bemused as a professor listening to the prattling of his freshman class”
perplexed by many conflicting situations or statements; filled with bewilderment
“obviously bemused by his questions”
unable to function; without help
1 of the 10 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for lost.
Synonyms, antonyms and the broader and narrower relations for lost come from Princeton University’s WordNet. Full sources and licences.