I find reports like this perplexing because I too am running Thunderbird on Windows 10 (and Linux!) and I haven't seen any significant changes, nor data loss. It's hard to see why users have such different experiences.

Could you provide a screen shot and describe what areas have changed?

in Windows, Snipping Tool will allow you to select what part of your screen or window you capture. Paste it into Paint, blob out anything personal that shouldn't be shared, save it as a PNG file and upload that PNG file to this forum with your reply.

Did anything dramatic happen, such as Thunderbird asking you to set up accounts from scratch? If so, that could be good news, since it means the old data may still be there.

Hi Zenos

The reply now shows your text in blue (it didn't use to) and also lots of source data seems to go with it but this wasn't so before. All is paragraphed now as before using Body Text? made things more unfriendly in presentation to recipients. I have screenshot my Thunderbird desktop, attached. On the left the folders under Admin - B01 onwards were nowhere to be found on the rebuild and nor were any Sent mails in the accounts (only the received Inbox mail items were recovered). I often moved Inbox or Sent emails to folders and these going back some years disappeared. Also, I have tried to underline and bolden this line n my email program but Ctrl B and Ctrl U don't work on this program anymore. If It tried to forward a reply to you I would / have had to change them to attachments rather than inline as the latter formats them to something almost unreadable (certainly receiver unfriendly) I can't frankly remember what happened in terms of / re set up; everything was such a dramatic flurry after Windows 10 managed to upgrade and destroy much just before Christmas - including deleting my Back up Cloud Drive program too... and I was in a blind rage of sorts because of so much data being misplaced or lost (with Microsoft of course)

Any help / guidance appreciated! MartynB

The usual reason for a sudden loss of formatting capabilities is that it has decided to default to plain text and not HTML.

So, do you still get the toolbar for underlining, bold etc? If not, go to Tools|Account Settings, select the account and under Composition & Addressing, make sure the "Compose messages in HTML format" checkbox is ticked. Repeat this for each account where you want to use these features. (I suspect this is not your problem, since you mention body text vs paragraph text. But we have to start somewhere.)

If you don't have the toolbar, another thing to check is that it is enabled. In a composition widow, go to View|Toolbars and tick the checkboxes there.

Note that you post your reply to this forum in your browser, so it seems to me that some of these comments ought to be aimed at your browser, not Thunderbird.

Thanks Zenos! I think we (you) have sorted it! Well done and best wishes..