Source: The Great Interpretation of Dreams (Ibn Sirin)
Breaking a key in a dream indicates difficulty in affairs and loss of help.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Breaking a key indicates despairing of a matter or loss of knowledge.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Breaking a key is interpreted as worry. Whoever sees his key break will be afflicted with worry.