What went wrong?

Nothing. The Win10 upgrade erased your Thunderbird profile, hence Thunderbird created a new one and then asked you to create an account.

Did you create a full backup of your Thunderbird profile prior to the Win10 upgrade? Or just parts of the profile, like mail files and address book?

Hello Christ1, I copied the complete C-disk, including all Thunderbird maps and files.

You copied the entire old C drive over a new Win10 system? I doubt that.

No, I copied C to my backup disk

You'll need to decide for yourself what's more annoying, continue with the new profile and import all the data from your old profile, or restore the old profile and import the data from the new profile. Personally I'd chose the latter option. https://support.mozilla.org/en-US/kb/profiles-tb#w_restoring-a-profile-backup http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

You could have avoided all the hassle if you'd have restored your profile right away (or would have asked here before messing things up).

It's very annoying indeed, especially because TB did not offer the opportunity to restore the profile after the upgrade to windows 10. TB only offered the possibility to enter a new account. Having said that, please tell me what I should do the next time this problem of not being able to see e-mails, occurs (i.c. how can I restore a profile instead of making a new account); I could not find any information on this. The problem will happen again, I'm sure as this is the third time it happened to me. The first times I solved the problem in a way I do not understand myself, now I want to know how it should be done.

I cannot yet tell if your last option is succesfull as the problem occurs on my son's computer and he is not living with me; it may take a while.

Last question: what is the difference between a profile and an e-mailaccount? How are different profiles shown in TB? I'm just a simple TB-user who chose TB because of it's simplicity but I begin to wonder if I made the right choice.

Your profile, in the context of Thunderbird, is a set of folders containing settings (including accounts), stored messages, address books, calendars, customizations, filters etc.

https://support.mozilla.org/en-US/kb/profiles-tb

So, you

1. Backup your profile.
2. Do the upgrade.
3. install Thunderbird and allow it to create a new profile.
4. Either add the new profile and use the profile manager to select it as the working profile, or,
5. overwrite the new empty profile with the old profile.

Multiple profiles are, IMHO, an unnecessary aggravation and complication. They are painful to switch between, and you can't move or share content between them easily. I'd go for 5.

I have used Thunderbird almost exclusively for several years so I am not well placed to comment on other email clients. I have Outlook at work, and I am supposed to use it but it feels awkward and restrictive to me. I wonder how (or if) do other email clients support this business of retaining data across a seismic upheaval such as an upgrade of OS? At least Thunderbird has a documented procedure to follow.

I still wonder why TB did not recognize the existing profile. Why take this detour of creating a new profile when the existing profile was still there.

So the next time TB does not show my e-mails I must not enter an e-mail-account (as TB asks) but instead look in TB for the existing profile (first pull-down menu, fourth choice from under) and select this other profile as the working profile??

I still wonder why TB did not recognize the existing profile. Why take this detour of creating a new profile when the existing profile was still there.

It wasn't, you erased it by upgrading to Win10. But this was explained to you already.

So the next time TB does not show my e-mails I must not enter an e-mail-account (as TB asks)

Exactly.

but instead look in TB for the existing profile (first pull-down menu, fourth choice from under) and select this other profile as the working profile??

No, you do need to restore your backed up profile. How to backup and restore a profile is explained in the article Xenos linked above. Read it.

This morning I visited my son to discuss the making of a new profile and to check TB. After starting TB, much to my suprprise, all e-mails of his former account were back including the e-mails from after the upgrade to Windows10. I asked him if he had done something special but that was not the case. He told me that the day after the upgrade, the then freshly made 2 accounts were gone and that he had tried to enter them again which was not possible because he did not have the necessary passwords. He cannot have restored the backup because I have the backup-disk. And now, today, all e-mails are back (still have to enter the second account again, did not yet have time for that). Must have been the fairies at work again, mustn't it. Anycase thanks for helping Christ and Zenos, I know what to do next time.

Can I please ask a question sort of relating to the above. I recently set up a new Thunderbird account on my new laptop. I didn't migrate any info from my old one as I wanted to clear out a load of junk anyway. I was then instructed after successfully using Thunderbird to do a Windows 10 Anniversary update. After completion one of the four email accounts I set up is visible but junk, draft, in box etc is missing along with the other three accounts. I had just set up my address book as well. When I go into Thunderbird it asks me for my email passwords because that is how I set it up but none of the information appears on the screen. If I go into options account settings all my accounts are still there. can someone help me restore the info or do I have to clear all the data and start again? can someone explain it simply please as I am not computer lit at all. Thank you. Julie

The first time my emails were missing also was after a Windows 10 update. The next day the emails were back, I cannot explain why. This may be the same in your case so try to waite for a day (or maybe a restart of windows). If this does not work I hope somebody else can be of help.