Verify your firewall settings and make sure Thunderbird isn't blocked.

Also take a look at these troubleshooting steps.

https://support.mozilla.org/en-US/kb/problems-sending-and-receiving-messages

Thank you for your answer. Of course before posting I checked the differetn relmated topics.. As I explained nothing changed between the moment Thunderbird worked and the moment it stopped working. I checked the firewall (Norton intenet security) and the firewall of my router (Netgear). Nothing relates to Thunderbird. I cans get my mail from another workqtation (ubuntu/evolution) with the same provider (same parameters). I just wonder if the last windows 8.1 updates could provoke this trouble... For the moment I cannot get rid of this.

What error message do you get?

Hi Christ No message. The green bar on the status bar keeps green for a while and nothing happens. It seems that I get the same problem during the installation of a new version of ccleaner: during the phase of the installation when ccleaner installer checks if updates are available on the web...this phase never stops... so the problem could be that some ports are closed.

Hi the problem is solved. I saw a small logo on the first Windows 8 connection window telling "no internet connection available". I connect to internet before opening a Windows session and after opening the windows session my Thunderbird works again as well as ccleaner update.. It seems that for this Windows 8.1 there are 2 kinds of internet connections...Mystery???