Right click the folder, select properties and then the repair button. That will rebuild the index of mail in the folder.

Matt, I did that rebuild you said, but this resulted the messages before march disappeared from Inbox, and I need them back. Could you help me again, with another solution? Thanks

Look in your anti virus program in it's logs etc for a file called inbox being deleted or quarantined for some reason. I would suggest looking around the date the problem manifested itself.

Matt, really the folder was in the quarantine of Norton Antivirus, and I succeeded in restore it to the folder of Thunderbird, and replace the empty folder. Thanks, Wagner