、。
She gave me a lovely watch, but I went and lost it.
Literal
She [topic-は] lovely watch [object-を] gave-me [but-が], I [topic-は] it [object-を] lost-completely.
素敵 ('lovely / wonderful / fantastic') is a na-adjective for warm aesthetic praise — slightly softer and more affectionate than すばらしい ('splendid / magnificent'). Its kanji 素敵 are 当て字 (ateji) — chosen for sound rather than meaning, giving the word a casual, even cute feel; it's frequently written in hiragana as すてき. Note the inclusion of explicit それ ('it') as the object of なくす — Japanese usually drops the pronoun when the referent is obvious, so spelling it out adds slight emphasis on what was lost. The rest of the structure mirrors a clean giving/receiving + ~てしまう regret frame.