。
I can't find my wristwatch.
Literal
Wristwatch [subject-が] not-find-[state-てない]-[explanatory-んです].
見つからない is the negative form of the intransitive 見つかる ('to be found' — Japanese frames searching with the intransitive, where English would use the active 'I can't find it'). ~ないんです is the casual contraction of ~ないのです, the explanatory polite form. The んです softens a bare negative into 'here's the situation' — natural when explaining a problem rather than just stating it.