I can no longer drag emails for filing in folders.

What prevents you from doing so?

From where to where exactly do you want to drag messages?

What's your account type - POP or IMAP?

Thank you for responding. I have a POP account. T-Bird page divided in two: on the right, Inbox, Outbox etc messages, just with the subject lines and in a chronological order, and on the left an archive, full of folders. I used to be able to drag any message to any folder on the left. Alternatively, I could cut or copy a message, and then past it into the appropriate folder. Neither is possible now; nothing happens when I undertake these actions.

Presumably your mail files for the source folder are corrupted. First of all create a backup of your Thunderbird profile folder. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup done prior to the corruption.

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird