Alot depends on the amount of immersion in water&muck, etc. Clean the immediate areas as best as possible. Dip and rinse in 90%-100% wood or isoproyl alcohol(don't smoke!) and then aloow to dry. Once dried, if possible remove PCB electronics bd. and check there for grime, etc., use soaked alcohol brush to clean pcb and then allow to dry. Don't use any other solvent(any auto product), that does attack plastic, remove stickers, labels(etc.) and can't dry quickly or leave stains. We use tricol(long name) in days past but can't be brought in large qualities. Most HDs have a small filter in order to allow atmospheric pressure to stabilze the HD innards, but it can be clogged or simply mucked-up. I suggest a real longtime to dry out like a sunlit window for a week or more, then allow to cool before use. If you doubt the pcb, then replace from a used or good HD of same model/type and proceed then. Of course you take your chances if all this works. Otherwise, if data is super critial then allow a pro at it, maybe insurance will cover that. Overall, HDs are pretty robust and can recover, even if outside is kaupt the insides can survive to varying degrees.