Mail - entire inbox of emails has gone missing

Hi - I have 2 Gmail accounts (IMAP) set up in Mail on my iMac, which have been working fine for a couple of years. This morning one of the inboxes is completely empty (should have been around 100 emails) and all the Mailboxes(50) I created for that account are empty too. It looks as if all the emails are now in the Archive folder (dating back to 2011). I've checked the Rules - there's nothing in there apart from for Apple News. Recovered Messages has about 10 random emails in but nothing from this particular Gmail account. This is an email I use for my business and the folders are for all of my client's projects so the thought of losing this is kind of scary. I could rebuild or restore from Time Machine back up, however, I'm not sure if this will restore all the emails in the correct folders or not? Any help would be really appreciated - thanks!