Source: The Great Interpretation of Dreams (Ibn Sirin)
Losing a key in a dream indicates difficulty in affairs and loss of help and sustenance.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Losing a key indicates loss of knowledge, money, or a spy.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Losing a key is interpreted as worry and difficulty in affairs.