There is no tool to de-compile an App Inventor apk to an aia file David. To continue to work on your Project, you
need your aia. Did you save one?
You said the aia file "disappeared from my project list." Did you save an aia somewhere or do you mean that the Project no longer appears in My Projects ? Could you
explain this please...ordinarily Projects do not 'disappear' unless the user actually deletes the Project. Are your 'other' Projects still there?
Do you have another App Inventor log in account. Are you using the same log in you used when you originally created the Project? If you have a second account, try
opening App Inventor using that account.
Regards,
Steve