Most likely a broken ward in the lock by: Webmaster
Is it a warded lock? Do you have a picture of the key or the lock? If it is warded (most likely), then perhaps it has broken off. In which case, try bending the end of a nail filer or related strong metal object, and sticking it in the lock and turning it. The bend will have to make the tip short enough to fit in the lock, but longer than the key since the key is not reaching the ward. But I can't say for certain without a picture of the lock or key.
