Once you forget the password for hidden chats, it is absolutely impossible to restore your hidden chats! But you can Reset Hidden Mode and all you hidden chats will be deleted!
How to reset the Hidden Mode?
Step 1: Open settings from the 'Three dot menu'
Step 2: Select 'Privacy'
Step 3: Select 'Hidden Mode'
Step 4: Select 'Reset Hidden Mode'
Step 5: Tap on 'Confirm'
Step 6: Now, you will taken back to home screen and you need to wait for next 30 Minutes. You will be able to see the counter on the home screen.
Step 7: After 30 minutes, tap on the red ribbon on the chat thread.
Step 8: Tap on 'Confirm' and your hidden chats will be 'Reset'