Question:

"Ophelia has lost her key"
or
"Ophelia lost her key"

Where is the the slight difference in the translation to japanese?

Answer:

I would translate;

オフィーリアは鍵を無くしてしまったのだ。(she has just lost her key)

オフィーリアは（以前）鍵を無くした。(uncertain when she lost her key, possible 'a long time ago')