Person's name no e-mail address wo nakushite shimatta/nakushichatta/nakushichattandayo.
Person's name no email address wo nakushita rashii no desu. (I seem to have lost....so a little formal).
Agreed with Dave on "no tokoro ni aru." A translation from one language to another where the translation is overly literal, grammatically incorrect, very awkward....what's the point even for the purpose of a test ??