Why is WinSCP window not showing after I start the program? Only a taskbar button appears.

WinSCP always starts on the monitor, where you have closed it the last time (if that monitor is still connected). It is possible that you have turned the monitor off meanwhile. On Windows 10, go to Settings > System > Display and check that you recognize all monitors listed there.

Alternatively, try pressing keyboard shortcuts Win+Shift+Left and Win+Shift+Right few times to try moving WinSCP window to other monitor.1 Do that while WinSCP application is active (by pressing its taskbar button first).

If none of this helps, you can try resetting windows layout by removing the following registry keys (while WinSCP is not running):

[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Interface\Commander]
[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Interface\Explorer]

and the following registry value:

[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Interface]
"LastMonitor"=dword:........