What OS are you using?

If you click on 'Write' and you see the Write icon in bottom Task Bar, this sounds like the window has opened but it is either: Not got focus, so it is hidden behind the main thunderbird interface or another opened window. or it has opened in minimised state, so is not behind other opened windows.

To test: Click on Write to open a new Write message. Minimise Thunderbird, so it shows as icon on bottom Task Bar. Can you see the open Write window? If yes, then the Write message window is not getting focus.

If no click on the Write icon in the Task Bar does it then open. If yes then the Write window is opening in a minimised state.

What results did you get?

Is it possible that you are using Win7 or Win8? Do you have the option to activate/ have focus on the window which the mouse is hovering over? I'm thinking that if you have this option selected then the main TB window may stay in focus because that is where the mouse pointer is pointing?

Second Test: Restart Thunderbird in Safe mode.

Help > Restart with Addons disabled

Now test: click on Write - did Write window open in the front in focus. If yes then it is likely that an addon is causing a conflict. You would need to test by gradually enabling addons/extensions until you find the one which is causing the problem.

Please report back on results.

In both thunderbird Safe mode and normal mode, does Ctrl+N open a new Write window in focus ?

Thank you Toad Hall.

It shows as minimised only. Windows 8 OS.

I disabled the Add-ons and it worked when restarted. So I can write and reply to emails. Phew! Relief.

Great!

That;s good news. now you need to restart thunderbird, then go to Tools > Addons disable all the addons and then enable one at a time to see when the error occurs. This will tell you which Addon is causing the problem.

Maybe the addon needs updating.