Talk to your email provider. They should have a way to reset your password.

My folder ....Appdata\Roaming\Thunderbird has 8.06 MB so my e-mails should be there.

8 MB isn't a lot, and there are other files than just mail in a profile. What size would you expect?

What happened to your existing profile anyway? Are you certain this is your old profile and not a new one?

Hi Christ1, Thank you for your very prompt response. I spoke to my service provider and they tried but were not able to help. I am getting e-mails at present from webmail.plus.net but there are only 40 e-mails in that InBox. My profile with 8.06 MB contains 146 files and 19 folders, so is probably my old profile though its creation date is given as 16 Sept 2016. I should be able to recover the actual old profile from backup if necessary but it doesn't help if I can't get nto Thunderbird. Why can't Thunderbird send me an email (which I can pick up from webmail) requesting password reset?

Why can't Thunderbird send me an email (which I can pick up from webmail) requesting password reset?

Thunderbird is an email client running on your computer. All it can do is to remember your password you set up on your email provider's server. You'd need to ask that question to your email provider.

I should be able to recover the actual old profile from backup

If you told Thunderbird to remember your password it would be stored in your profile. So restoring your backed up profile would also restore the password.

But then if you are really using your existing profile right now you shouldn't have to do that.