Column cannot be completely hidden this way. But it can be very narrow.
If there's a "hidden" column that is too narrow between two other columns (or next to first/last column), you will see your mouse cursor to change to two vertical lines with arrows pointing left and right (as opposite to single bold line) when you hover the cursor over columns separator. Just hold down a left mouse button to resize the column back.
Or just close WinSCP, delete below registry values, and restart it:
[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Interface\Commander\LocalPanel]
[HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\Interface\Commander\RemotePanel]