Is the Folder Pane visible ?

Menu icon > Options > Layout > select 'Folder Pane'

Firstly, in response to your question, I am guessing the folder pane is the column on the left where my local folders and sub folders are. ( I am not experienced).....The column is there but no folders whatever. The other part where the messages are usually listed with a preview section beneath is just one area also blank. the menu icon is greyed out.

Secondly, I appear to have solved the problem by cheating/doing the wrong thing and getting the right result. I have a spare computer that is setup exactly the same as my main computer. I fired that up and found that Thunderbird showed everything as normal.(No update) I copied all Thunderbird folders and files etc from Programs files(x86), App Data Local and Roaming then pasted them in to my main machine. This Thunderbird now started up as normal and showed all new messages. (not sure how long this would last) The second machine then updated Thunderbird and as before I had a blank page and nothing else. I copied my files back to that machine and again I have Thunderbird running normally there too. I have since checked and both machines say that they have an up to date Thunderbird and appear to be working normally.

Submitted for your information. Alan WP

I assume the account type is IMAP and your firewall simply does not recognize Thunderbird with it's new version number. This is particularly common for Nortons users, but most security products with a software firewall are prone to doing in to some extent.

Solution, fix the firewall component to allow the new product version, or get a product that does not block without first asking.

Thanks for your input Matt, You are probably correct (I don't know) As I stated above the action I took appeared to fix the problem and I can confirm, a week later and over a hundred emails, that everything is working as normal. Not touching anything else now. I can also confirm that I use Kaspersky internet security (paid version) and it does notify me of any action taken.

My emails got completely wiped out and Thunderbird is asking for the passwords, etc as if it is a completely new install, when all it was supposed to do was UPDATE. Not REPLACE!!! I really don't know what to do! Since when is this acceptable?

I have NO IDEA how to fix a Firewall component! AVG is installed. I looked through their help. USELESS. Possibly worse than finding applicable info for Thunderbird. This was the only thing close to my problem.

I turned off all virus protection and still have my issue...

StampedeQueen This question has been solved. Firewalls can also prevent accessing internet to get emails and AV products that scan folders can delete whole folders. The solution in this instance was to copy a backup into the correct location to reinstate everything. If this is not a solution that worked for you then I strongly recommend you create your own question as all your comments are going to the person who originally asked this question, which may not be what you intend nor what the original postee desires. Your issue may be entirely different and needs investigating in it's own right in it's own question.