Further information which may help find a solution. When I load up Thunderbird it first says 'building summary file for inbox' this then changes to 'determining which messages to index' and finally it tells me that my inbox is full and that I should delete some messages and compact the files. I have only about 10 messages in my inbox and have deleted all sent files and emptied the deleted file to no effect. There are far less files now than when Thunderbird was operating normally earlier today.
I have a number of saved emails that are very important and cannot be lost, plus all my contacts are in the address book. Is there a way of saving all this data in the event that I need to uninstall Thunderbird and start with a fresh install.
One other problem is that I can no longer remember my password for one of my email accounts. Is there a way of recovering this from Thunderbird?