My inbox won't let me open it and I have no idea why?
There is a padlock symbol over my email address! I probably did something that has caused this because I know very little about settings and all of the operation workings! I can see that emails are still arriving, but I have no way of reading them.
Alla svar (2)
re :There is a padlock symbol over my email address!
A padlock next to the account means Thunderbird has that account set up to use encryption (SSL/TLS) to connect to the remote server. Good to hear - it simply means that it is using a safe connection - I would more concerned if it wasn't there :) So do not worry about that.
re :I can see that emails are still arriving, but I have no way of reading them. I presume you mean it says 'downloading x emails' in Status bar', but you do not say what is in the Inbox - old messages - no messages etc. Can you see your folders in the Folder pane ? I'm assuming you can. You do not mention whether you have created any Message Filters which could be moving emails into various folders etc. Maybe you are not showing unread mail or perhaps you are using threads and the email is located further down within a thread. Maybe you have a filter still enabled, so it is only showing what the filter allows.
so check some settings: Menu icon > Folders > All Menu icon > View > Messages > All Menu icon > View > Threads > All Menu icon > View > Sort by > select these: 'Date', 'Descending', 'unthreaded'
Make sure no filters are selected in the 'Quick Filter Toolbar'