Exit Thunderbird if running. Make sure Thunderbird is not running. right click on bottom Task Bar and select 'Task Manager'. Under Applications see if 'Mozilla thunderbird is running. if yes, then select it and clickon 'End task' check 'Processes' tab make sure thunderbird.exe is not running. if yes, then select Thunderbird and click on 'End Process'. See if the 'parent.lock ' file is causing an issue. Access your Thunderbird profile name folder. it is located here: * C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ 'Profile name' folder is typically called 'xxxxxxxx.default' where the x's are letters and numbers. Note: The AppData folder is folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders". In 'profile name' folder, scroll down delete the 'parent.lock' file. start Thunderbird. Report back on results.