no longer seeing emails

Did you activate the "Quick Filter" toolbar in your Inbox folder?

If you have enabled a filter on the Inbox then it can alter the display. check you have the following set up. Via the 'Menu Bar' toolbar

• 'View' > 'Folders' > only select 'All folders'
• 'View' > 'Threads' > only select 'All'

If you see any of the 'icons' displaying a coloured strip in the 'Quick Filter' toolbar ? It might be the 'pin' icon or any of the others which has applied a filter set on another folder. If yes, then click on the icon to remove that filter and it also removes the highlight.

Not seeing new mail may mean another problem. You would need to logon to the webmail account via a browser and find out if new mail has actually been received into webmail account. If you have no new mail on server then check - have you used up your server quota. Basically, if you have used up all the allowed server space and it's full, the server cannot receive any new mail until you have created some space by deleteing old mail off server. Any emails sent to you get returned to sender as undeliverable.

If you use IMAP accounts in Thunderbird then you would need to get a 'Copy' of important mail into the 'Local Folders' account before you delete off the server because Imap accounts can only display whatever is on the server.

If you use Google then please note server quota is not just emails, it also includes calendar and Google photos.

If you use POP and gmail: Gmail can get to be a pain when it comes to Pop accounts because they cannot put deleted mail into the server Trash folder. That means everything is still in the 'All Mail' fodler on server. So gmail ends up slowly restricting what can be downloaded thus forcing you to logon to webmail account, access 'All Mail' folder and delete a load of emails off the server. Then downloads will commence again.