My G Key has started falling off too.
I bought my Mac Pro in October 2014, last year, and it's treated with the utmost care and has never received a knock once - not even harsh language.
I also have a kuzzy cover and I then put it inside a Twelve South BookBook Hard Leather Cover - To be sure, to be sure.
I don't recommend either of them by the way.
-The Kuzzy plastic deteriorated and the rubber feet fell off.
-The BookBook has warped from the Laptop heat because the Leather and Lining are two different materials. This has caused the cover to bow. The elastics that hold the cover to my Laptop screen keep coming off.
Anyway - Long story short. The fact that you may have dropped your Laptop is decidedly irrelevant. The key would have probably come off anyway.
I think Apple have recently redesigned the keys anyway.