I would assume you are seeing some sort of error message, what is it?

A small box appears with the message

Unable to login at server Probably wrong configuration user name or password

I have tried both my Full name and my first name and surname. My password is correct as I have it written down. David

re :Tried Setup Your Existing E-mail Address. This recognised my imap account settings, but error message came up saying incoming server already exists.

That's because it does exist. You already have a imap mail account set up for that email address and you say you are still able to send. So there is no fault with that message.

re :Wanted to block sender on my main account ..... using a filter. Found this deleted all messages in my inbox

Clearly, you did something very wrong. Either you chose to block yourself or set up a condition that would apply to any email. Did you create the 'Message filter' in Thunderbird? Did you create the Message Filter on the server via webmail account ? You say you deleted the problem Filter. At this point - Have you removed/disabled ALL Message Filters? If no, then disable all the Message Filters you have set up for that account.

re : Error Message : Unable to login at server - Probably wrong configuration user name or password

It could be a typo error with User Name or password or perhaps they are ok but not with whatever server settings you have got set up. So it's necessary to check all.

re :I have tried both my Full name and my first name and surname.

In Thunderbird's  'Account Settings' > 'Server Settings' for the account/email address

'User Name' is usually your full email address, but on a few occasions it will be the first part of email address - the bit before the @ sign. Depends upon the server.

Perhaps it would be a good idea to post some information about the current state of the server settings. Then we can offer better information rather than guessing. In Thunderbird Menu icon > Help > More Troubleshooting Information Scroll to 'Mail and News Accounts' section Copy all the data in that section and paste it into this forum question 'Post a Reply' text box.

upload a jpeg image showing what you see in: 'Account Settings' > 'Server Settings' for the account