With iMessage turned off, will I receive messages once I turn it back on?

If the iMessage is turned off then you would either receive it through your Apple ID email account or will receive an SMS depending on what the sender has used for sending iMessage; Apple ID, or Phone number respectively.

If the sender has no SMS service enabled through their carrier then messages will never get sent from their device if they use your phone number.

Once you enable iMessage what you will receive are fresh messages sent after you have enabled the iMessage on your device and not the old ones sent earlier to your enabling it.