Can anyone let me know what to do in the case of a key investigator being uncontactable at the time of manuscript submission. Most journals ask for a signature and address of all authors and yet we can't track our investigator down.
This is in itself is a weird situation in which journal won't be able to help you. You can request the journal to give you special permission for submission or you can find a journal where signatures are not needed.
In some cases, even I was not is a position to get the signatures of all the authors (I knew the address though), I opted not to submit in such journals. Instead I searched another journal where corresponding author signatures are sufficient for submission and subsequent proceedings.