Hmmm, that would be odd. Let's troubleshoot and try to fix this for you. It sounds like Thunderbird is set up on your machine for the Unified Folders view. (In that view, Inbox folders from different accounts are grouped together at the top of the folder list, so it can look like one account has disappeared.) To check if this is the issue: In Thunderbird, click View in the top menu. Select Folders → then click 'All Folders'. This will show each account separately in the left-hand panel. Hopefully, this helps. If you prefer to keep using Unified Folders, you should be able to expand the “Inbox” group by clicking the small arrow next to it to see all accounts inside.